Fix a conceptual error in my patch of 2007-10-26 that avoided considering

[postgresql] / src / include / nodes / relation.h

/*-------------------------------------------------------------------------
 *
 * relation.h
 *        Definitions for planner's internal data structures.
 *
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.154 2008/01/11 04:02:18 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#ifndef RELATION_H
#define RELATION_H

#include "access/sdir.h"
#include "nodes/bitmapset.h"
#include "nodes/params.h"
#include "nodes/parsenodes.h"
#include "storage/block.h"


/*
 * Relids
 *              Set of relation identifiers (indexes into the rangetable).
 */
typedef Bitmapset *Relids;

/*
 * When looking for a "cheapest path", this enum specifies whether we want
 * cheapest startup cost or cheapest total cost.
 */
typedef enum CostSelector
{
        STARTUP_COST, TOTAL_COST
} CostSelector;

/*
 * The cost estimate produced by cost_qual_eval() includes both a one-time
 * (startup) cost, and a per-tuple cost.
 */
typedef struct QualCost
{
        Cost            startup;                /* one-time cost */
        Cost            per_tuple;              /* per-evaluation cost */
} QualCost;


/*----------
 * PlannerGlobal
 *              Global information for planning/optimization
 *
 * PlannerGlobal holds state for an entire planner invocation; this state
 * is shared across all levels of sub-Queries that exist in the command being
 * planned.
 *----------
 */
typedef struct PlannerGlobal
{
        NodeTag         type;

        ParamListInfo boundParams;      /* Param values provided to planner() */

        List       *paramlist;          /* to keep track of cross-level Params */

        List       *subplans;           /* Plans for SubPlan nodes */

        List       *subrtables;         /* Rangetables for SubPlan nodes */

        Bitmapset  *rewindPlanIDs;      /* indices of subplans that require REWIND */

        List       *finalrtable;        /* "flat" rangetable for executor */

        List       *relationOids;       /* OIDs of relations the plan depends on */

        bool            transientPlan;  /* redo plan when TransactionXmin changes? */
} PlannerGlobal;

/* macro for fetching the Plan associated with a SubPlan node */
#define planner_subplan_get_plan(root, subplan) \
        ((Plan *) list_nth((root)->glob->subplans, (subplan)->plan_id - 1))


/*----------
 * PlannerInfo
 *              Per-query information for planning/optimization
 *
 * This struct is conventionally called "root" in all the planner routines.
 * It holds links to all of the planner's working state, in addition to the
 * original Query.      Note that at present the planner extensively modifies
 * the passed-in Query data structure; someday that should stop.
 *----------
 */
typedef struct PlannerInfo
{
        NodeTag         type;

        Query      *parse;                      /* the Query being planned */

        PlannerGlobal *glob;            /* global info for current planner run */

        Index           query_level;    /* 1 at the outermost Query */

        /*
         * simple_rel_array holds pointers to "base rels" and "other rels" (see
         * comments for RelOptInfo for more info).      It is indexed by rangetable
         * index (so entry 0 is always wasted).  Entries can be NULL when an RTE
         * does not correspond to a base relation, such as a join RTE or an
         * unreferenced view RTE; or if the RelOptInfo hasn't been made yet.
         */
        struct RelOptInfo **simple_rel_array;           /* All 1-rel RelOptInfos */
        int                     simple_rel_array_size;  /* allocated size of array */

        /*
         * simple_rte_array is the same length as simple_rel_array and holds
         * pointers to the associated rangetable entries.  This lets us avoid
         * rt_fetch(), which can be a bit slow once large inheritance sets have
         * been expanded.
         */
        RangeTblEntry **simple_rte_array;       /* rangetable as an array */

        /*
         * join_rel_list is a list of all join-relation RelOptInfos we have
         * considered in this planning run.  For small problems we just scan the
         * list to do lookups, but when there are many join relations we build a
         * hash table for faster lookups.  The hash table is present and valid
         * when join_rel_hash is not NULL.      Note that we still maintain the list
         * even when using the hash table for lookups; this simplifies life for
         * GEQO.
         */
        List       *join_rel_list;      /* list of join-relation RelOptInfos */
        struct HTAB *join_rel_hash; /* optional hashtable for join relations */

        List       *resultRelations;    /* integer list of RT indexes, or NIL */

        List       *returningLists; /* list of lists of TargetEntry, or NIL */

        List       *init_plans;         /* init subplans for query */

        List       *eq_classes;         /* list of active EquivalenceClasses */

        List       *canon_pathkeys; /* list of "canonical" PathKeys */

        List       *left_join_clauses;          /* list of RestrictInfos for
                                                                                 * mergejoinable outer join clauses
                                                                                 * w/nonnullable var on left */

        List       *right_join_clauses;         /* list of RestrictInfos for
                                                                                 * mergejoinable outer join clauses
                                                                                 * w/nonnullable var on right */

        List       *full_join_clauses;          /* list of RestrictInfos for
                                                                                 * mergejoinable full join clauses */

        List       *oj_info_list;       /* list of OuterJoinInfos */

        List       *in_info_list;       /* list of InClauseInfos */

        List       *append_rel_list;    /* list of AppendRelInfos */

        List       *query_pathkeys; /* desired pathkeys for query_planner(), and
                                                                 * actual pathkeys afterwards */

        List       *group_pathkeys; /* groupClause pathkeys, if any */
        List       *sort_pathkeys;      /* sortClause pathkeys, if any */

        List       *initial_rels;       /* RelOptInfos we are now trying to join */

        MemoryContext planner_cxt;      /* context holding PlannerInfo */

        double          total_table_pages;              /* # of pages in all tables of query */

        double          tuple_fraction; /* tuple_fraction passed to query_planner */

        bool            hasJoinRTEs;    /* true if any RTEs are RTE_JOIN kind */
        bool            hasOuterJoins;  /* true if any RTEs are outer joins */
        bool            hasHavingQual;  /* true if havingQual was non-null */
        bool            hasPseudoConstantQuals; /* true if any RestrictInfo has
                                                                                 * pseudoconstant = true */
} PlannerInfo;


/*
 * In places where it's known that simple_rte_array[] must have been prepared
 * already, we just index into it to fetch RTEs.  In code that might be
 * executed before or after entering query_planner(), use this macro.
 */
#define planner_rt_fetch(rti, root) \
        ((root)->simple_rte_array ? (root)->simple_rte_array[rti] : \
         rt_fetch(rti, (root)->parse->rtable))


/*----------
 * RelOptInfo
 *              Per-relation information for planning/optimization
 *
 * For planning purposes, a "base rel" is either a plain relation (a table)
 * or the output of a sub-SELECT or function that appears in the range table.
 * In either case it is uniquely identified by an RT index.  A "joinrel"
 * is the joining of two or more base rels.  A joinrel is identified by
 * the set of RT indexes for its component baserels.  We create RelOptInfo
 * nodes for each baserel and joinrel, and store them in the PlannerInfo's
 * simple_rel_array and join_rel_list respectively.
 *
 * Note that there is only one joinrel for any given set of component
 * baserels, no matter what order we assemble them in; so an unordered
 * set is the right datatype to identify it with.
 *
 * We also have "other rels", which are like base rels in that they refer to
 * single RT indexes; but they are not part of the join tree, and are given
 * a different RelOptKind to identify them.
 *
 * Currently the only kind of otherrels are those made for member relations
 * of an "append relation", that is an inheritance set or UNION ALL subquery.
 * An append relation has a parent RTE that is a base rel, which represents
 * the entire append relation.  The member RTEs are otherrels.  The parent
 * is present in the query join tree but the members are not.  The member
 * RTEs and otherrels are used to plan the scans of the individual tables or
 * subqueries of the append set; then the parent baserel is given an Append
 * plan comprising the best plans for the individual member rels.  (See
 * comments for AppendRelInfo for more information.)
 *
 * At one time we also made otherrels to represent join RTEs, for use in
 * handling join alias Vars.  Currently this is not needed because all join
 * alias Vars are expanded to non-aliased form during preprocess_expression.
 *
 * Parts of this data structure are specific to various scan and join
 * mechanisms.  It didn't seem worth creating new node types for them.
 *
 *              relids - Set of base-relation identifiers; it is a base relation
 *                              if there is just one, a join relation if more than one
 *              rows - estimated number of tuples in the relation after restriction
 *                         clauses have been applied (ie, output rows of a plan for it)
 *              width - avg. number of bytes per tuple in the relation after the
 *                              appropriate projections have been done (ie, output width)
 *              reltargetlist - List of Var nodes for the attributes we need to
 *                                              output from this relation (in no particular order)
 *                                              NOTE: in a child relation, may contain RowExprs
 *              pathlist - List of Path nodes, one for each potentially useful
 *                                 method of generating the relation
 *              cheapest_startup_path - the pathlist member with lowest startup cost
 *                                                              (regardless of its ordering)
 *              cheapest_total_path - the pathlist member with lowest total cost
 *                                                        (regardless of its ordering)
 *              cheapest_unique_path - for caching cheapest path to produce unique
 *                                                         (no duplicates) output from relation
 *
 * If the relation is a base relation it will have these fields set:
 *
 *              relid - RTE index (this is redundant with the relids field, but
 *                              is provided for convenience of access)
 *              rtekind - distinguishes plain relation, subquery, or function RTE
 *              min_attr, max_attr - range of valid AttrNumbers for rel
 *              attr_needed - array of bitmapsets indicating the highest joinrel
 *                              in which each attribute is needed; if bit 0 is set then
 *                              the attribute is needed as part of final targetlist
 *              attr_widths - cache space for per-attribute width estimates;
 *                                        zero means not computed yet
 *              indexlist - list of IndexOptInfo nodes for relation's indexes
 *                                      (always NIL if it's not a table)
 *              pages - number of disk pages in relation (zero if not a table)
 *              tuples - number of tuples in relation (not considering restrictions)
 *              subplan - plan for subquery (NULL if it's not a subquery)
 *              subrtable - rangetable for subquery (NIL if it's not a subquery)
 *
 *              Note: for a subquery, tuples and subplan are not set immediately
 *              upon creation of the RelOptInfo object; they are filled in when
 *              set_base_rel_pathlist processes the object.
 *
 *              For otherrels that are appendrel members, these fields are filled
 *              in just as for a baserel.
 *
 * The presence of the remaining fields depends on the restrictions
 * and joins that the relation participates in:
 *
 *              baserestrictinfo - List of RestrictInfo nodes, containing info about
 *                                      each non-join qualification clause in which this relation
 *                                      participates (only used for base rels)
 *              baserestrictcost - Estimated cost of evaluating the baserestrictinfo
 *                                      clauses at a single tuple (only used for base rels)
 *              joininfo  - List of RestrictInfo nodes, containing info about each
 *                                      join clause in which this relation participates (but
 *                                      note this excludes clauses that might be derivable from
 *                                      EquivalenceClasses)
 *              has_eclass_joins - flag that EquivalenceClass joins are possible
 *              index_outer_relids - only used for base rels; set of outer relids
 *                                      that participate in indexable joinclauses for this rel
 *              index_inner_paths - only used for base rels; list of InnerIndexscanInfo
 *                                      nodes showing best indexpaths for various subsets of
 *                                      index_outer_relids.
 *
 * Note: Keeping a restrictinfo list in the RelOptInfo is useful only for
 * base rels, because for a join rel the set of clauses that are treated as
 * restrict clauses varies depending on which sub-relations we choose to join.
 * (For example, in a 3-base-rel join, a clause relating rels 1 and 2 must be
 * treated as a restrictclause if we join {1} and {2 3} to make {1 2 3}; but
 * if we join {1 2} and {3} then that clause will be a restrictclause in {1 2}
 * and should not be processed again at the level of {1 2 3}.)  Therefore,
 * the restrictinfo list in the join case appears in individual JoinPaths
 * (field joinrestrictinfo), not in the parent relation.  But it's OK for
 * the RelOptInfo to store the joininfo list, because that is the same
 * for a given rel no matter how we form it.
 *
 * We store baserestrictcost in the RelOptInfo (for base relations) because
 * we know we will need it at least once (to price the sequential scan)
 * and may need it multiple times to price index scans.
 *----------
 */
typedef enum RelOptKind
{
        RELOPT_BASEREL,
        RELOPT_JOINREL,
        RELOPT_OTHER_MEMBER_REL
} RelOptKind;

typedef struct RelOptInfo
{
        NodeTag         type;

        RelOptKind      reloptkind;

        /* all relations included in this RelOptInfo */
        Relids          relids;                 /* set of base relids (rangetable indexes) */

        /* size estimates generated by planner */
        double          rows;                   /* estimated number of result tuples */
        int                     width;                  /* estimated avg width of result tuples */

        /* materialization information */
        List       *reltargetlist;      /* needed Vars */
        List       *pathlist;           /* Path structures */
        struct Path *cheapest_startup_path;
        struct Path *cheapest_total_path;
        struct Path *cheapest_unique_path;

        /* information about a base rel (not set for join rels!) */
        Index           relid;
        RTEKind         rtekind;                /* RELATION, SUBQUERY, or FUNCTION */
        AttrNumber      min_attr;               /* smallest attrno of rel (often <0) */
        AttrNumber      max_attr;               /* largest attrno of rel */
        Relids     *attr_needed;        /* array indexed [min_attr .. max_attr] */
        int32      *attr_widths;        /* array indexed [min_attr .. max_attr] */
        List       *indexlist;
        BlockNumber pages;
        double          tuples;
        struct Plan *subplan;           /* if subquery */
        List       *subrtable;          /* if subquery */

        /* used by various scans and joins: */
        List       *baserestrictinfo;           /* RestrictInfo structures (if base
                                                                                 * rel) */
        QualCost        baserestrictcost;               /* cost of evaluating the above */
        List       *joininfo;           /* RestrictInfo structures for join clauses
                                                                 * involving this rel */
        bool            has_eclass_joins;               /* T means joininfo is incomplete */

        /* cached info about inner indexscan paths for relation: */
        Relids          index_outer_relids;             /* other relids in indexable join
                                                                                 * clauses */
        List       *index_inner_paths;          /* InnerIndexscanInfo nodes */

        /*
         * Inner indexscans are not in the main pathlist because they are not
         * usable except in specific join contexts.  We use the index_inner_paths
         * list just to avoid recomputing the best inner indexscan repeatedly for
         * similar outer relations.  See comments for InnerIndexscanInfo.
         */
} RelOptInfo;

/*
 * IndexOptInfo
 *              Per-index information for planning/optimization
 *
 *              Prior to Postgres 7.0, RelOptInfo was used to describe both relations
 *              and indexes, but that created confusion without actually doing anything
 *              useful.  So now we have a separate IndexOptInfo struct for indexes.
 *
 *              opfamily[], indexkeys[], opcintype[], fwdsortop[], revsortop[],
 *              and nulls_first[] each have ncolumns entries.
 *              Note: for historical reasons, the opfamily array has an extra entry
 *              that is always zero.  Some code scans until it sees a zero entry,
 *              rather than looking at ncolumns.
 *
 *              Zeroes in the indexkeys[] array indicate index columns that are
 *              expressions; there is one element in indexprs for each such column.
 *
 *              For an unordered index, the sortop arrays contains zeroes.      Note that
 *              fwdsortop[] and nulls_first[] describe the sort ordering of a forward
 *              indexscan; we can also consider a backward indexscan, which will
 *              generate sort order described by revsortop/!nulls_first.
 *
 *              The indexprs and indpred expressions have been run through
 *              prepqual.c and eval_const_expressions() for ease of matching to
 *              WHERE clauses. indpred is in implicit-AND form.
 */
typedef struct IndexOptInfo
{
        NodeTag         type;

        Oid                     indexoid;               /* OID of the index relation */
        RelOptInfo *rel;                        /* back-link to index's table */

        /* statistics from pg_class */
        BlockNumber pages;                      /* number of disk pages in index */
        double          tuples;                 /* number of index tuples in index */

        /* index descriptor information */
        int                     ncolumns;               /* number of columns in index */
        Oid                *opfamily;           /* OIDs of operator families for columns */
        int                *indexkeys;          /* column numbers of index's keys, or 0 */
        Oid                *opcintype;          /* OIDs of opclass declared input data types */
        Oid                *fwdsortop;          /* OIDs of sort operators for each column */
        Oid                *revsortop;          /* OIDs of sort operators for backward scan */
        bool       *nulls_first;        /* do NULLs come first in the sort order? */
        Oid                     relam;                  /* OID of the access method (in pg_am) */

        RegProcedure amcostestimate;    /* OID of the access method's cost fcn */

        List       *indexprs;           /* expressions for non-simple index columns */
        List       *indpred;            /* predicate if a partial index, else NIL */

        bool            predOK;                 /* true if predicate matches query */
        bool            unique;                 /* true if a unique index */
        bool            amoptionalkey;  /* can query omit key for the first column? */
        bool            amsearchnulls;  /* can AM search for NULL index entries? */
} IndexOptInfo;


/*
 * EquivalenceClasses
 *
 * Whenever we can determine that a mergejoinable equality clause A = B is
 * not delayed by any outer join, we create an EquivalenceClass containing
 * the expressions A and B to record this knowledge.  If we later find another
 * equivalence B = C, we add C to the existing EquivalenceClass; this may
 * require merging two existing EquivalenceClasses.  At the end of the qual
 * distribution process, we have sets of values that are known all transitively
 * equal to each other, where "equal" is according to the rules of the btree
 * operator family(s) shown in ec_opfamilies.  (We restrict an EC to contain
 * only equalities whose operators belong to the same set of opfamilies.  This
 * could probably be relaxed, but for now it's not worth the trouble, since
 * nearly all equality operators belong to only one btree opclass anyway.)
 *
 * We also use EquivalenceClasses as the base structure for PathKeys, letting
 * us represent knowledge about different sort orderings being equivalent.
 * Since every PathKey must reference an EquivalenceClass, we will end up
 * with single-member EquivalenceClasses whenever a sort key expression has
 * not been equivalenced to anything else.      It is also possible that such an
 * EquivalenceClass will contain a volatile expression ("ORDER BY random()"),
 * which is a case that can't arise otherwise since clauses containing
 * volatile functions are never considered mergejoinable.  We mark such
 * EquivalenceClasses specially to prevent them from being merged with
 * ordinary EquivalenceClasses.  Also, for volatile expressions we have
 * to be careful to match the EquivalenceClass to the correct targetlist
 * entry: consider SELECT random() AS a, random() AS b ... ORDER BY b,a.
 * So we record the SortGroupRef of the originating sort clause.
 *
 * We allow equality clauses appearing below the nullable side of an outer join
 * to form EquivalenceClasses, but these have a slightly different meaning:
 * the included values might be all NULL rather than all the same non-null
 * values.      See src/backend/optimizer/README for more on that point.
 *
 * NB: if ec_merged isn't NULL, this class has been merged into another, and
 * should be ignored in favor of using the pointed-to class.
 */
typedef struct EquivalenceClass
{
        NodeTag         type;

        List       *ec_opfamilies;      /* btree operator family OIDs */
        List       *ec_members;         /* list of EquivalenceMembers */
        List       *ec_sources;         /* list of generating RestrictInfos */
        List       *ec_derives;         /* list of derived RestrictInfos */
        Relids          ec_relids;              /* all relids appearing in ec_members */
        bool            ec_has_const;   /* any pseudoconstants in ec_members? */
        bool            ec_has_volatile;        /* the (sole) member is a volatile expr */
        bool            ec_below_outer_join;    /* equivalence applies below an OJ */
        bool            ec_broken;              /* failed to generate needed clauses? */
        Index           ec_sortref;             /* originating sortclause label, or 0 */
        struct EquivalenceClass *ec_merged; /* set if merged into another EC */
} EquivalenceClass;

/*
 * If an EC contains a const and isn't below-outer-join, any PathKey depending
 * on it must be redundant, since there's only one possible value of the key.
 */
#define EC_MUST_BE_REDUNDANT(eclass)  \
        ((eclass)->ec_has_const && !(eclass)->ec_below_outer_join)

/*
 * EquivalenceMember - one member expression of an EquivalenceClass
 *
 * em_is_child signifies that this element was built by transposing a member
 * for an inheritance parent relation to represent the corresponding expression
 * on an inheritance child.  The element should be ignored for all purposes
 * except constructing inner-indexscan paths for the child relation.  (Other
 * types of join are driven from transposed joininfo-list entries.)  Note
 * that the EC's ec_relids field does NOT include the child relation.
 *
 * em_datatype is usually the same as exprType(em_expr), but can be
 * different when dealing with a binary-compatible opfamily; in particular
 * anyarray_ops would never work without this.  Use em_datatype when
 * looking up a specific btree operator to work with this expression.
 */
typedef struct EquivalenceMember
{
        NodeTag         type;

        Expr       *em_expr;            /* the expression represented */
        Relids          em_relids;              /* all relids appearing in em_expr */
        bool            em_is_const;    /* expression is pseudoconstant? */
        bool            em_is_child;    /* derived version for a child relation? */
        Oid                     em_datatype;    /* the "nominal type" used by the opfamily */
} EquivalenceMember;

/*
 * PathKeys
 *
 * The sort ordering of a path is represented by a list of PathKey nodes.
 * An empty list implies no known ordering.  Otherwise the first item
 * represents the primary sort key, the second the first secondary sort key,
 * etc.  The value being sorted is represented by linking to an
 * EquivalenceClass containing that value and including pk_opfamily among its
 * ec_opfamilies.  This is a convenient method because it makes it trivial
 * to detect equivalent and closely-related orderings.  (See optimizer/README
 * for more information.)
 *
 * Note: pk_strategy is either BTLessStrategyNumber (for ASC) or
 * BTGreaterStrategyNumber (for DESC).  We assume that all ordering-capable
 * index types will use btree-compatible strategy numbers.
 */

typedef struct PathKey
{
        NodeTag         type;

        EquivalenceClass *pk_eclass;    /* the value that is ordered */
        Oid                     pk_opfamily;    /* btree opfamily defining the ordering */
        int                     pk_strategy;    /* sort direction (ASC or DESC) */
        bool            pk_nulls_first; /* do NULLs come before normal values? */
} PathKey;

/*
 * Type "Path" is used as-is for sequential-scan paths.  For other
 * path types it is the first component of a larger struct.
 *
 * Note: "pathtype" is the NodeTag of the Plan node we could build from this
 * Path.  It is partially redundant with the Path's NodeTag, but allows us
 * to use the same Path type for multiple Plan types where there is no need
 * to distinguish the Plan type during path processing.
 */

typedef struct Path
{
        NodeTag         type;

        NodeTag         pathtype;               /* tag identifying scan/join method */

        RelOptInfo *parent;                     /* the relation this path can build */

        /* estimated execution costs for path (see costsize.c for more info) */
        Cost            startup_cost;   /* cost expended before fetching any tuples */
        Cost            total_cost;             /* total cost (assuming all tuples fetched) */

        List       *pathkeys;           /* sort ordering of path's output */
        /* pathkeys is a List of PathKey nodes; see above */
} Path;

/*----------
 * IndexPath represents an index scan over a single index.
 *
 * 'indexinfo' is the index to be scanned.
 *
 * 'indexclauses' is a list of index qualification clauses, with implicit
 * AND semantics across the list.  Each clause is a RestrictInfo node from
 * the query's WHERE or JOIN conditions.
 *
 * 'indexquals' has the same structure as 'indexclauses', but it contains
 * the actual indexqual conditions that can be used with the index.
 * In simple cases this is identical to 'indexclauses', but when special
 * indexable operators appear in 'indexclauses', they are replaced by the
 * derived indexscannable conditions in 'indexquals'.
 *
 * 'isjoininner' is TRUE if the path is a nestloop inner scan (that is,
 * some of the index conditions are join rather than restriction clauses).
 * Note that the path costs will be calculated differently from a plain
 * indexscan in this case, and in addition there's a special 'rows' value
 * different from the parent RelOptInfo's (see below).
 *
 * 'indexscandir' is one of:
 *              ForwardScanDirection: forward scan of an ordered index
 *              BackwardScanDirection: backward scan of an ordered index
 *              NoMovementScanDirection: scan of an unordered index, or don't care
 * (The executor doesn't care whether it gets ForwardScanDirection or
 * NoMovementScanDirection for an indexscan, but the planner wants to
 * distinguish ordered from unordered indexes for building pathkeys.)
 *
 * 'indextotalcost' and 'indexselectivity' are saved in the IndexPath so that
 * we need not recompute them when considering using the same index in a
 * bitmap index/heap scan (see BitmapHeapPath).  The costs of the IndexPath
 * itself represent the costs of an IndexScan plan type.
 *
 * 'rows' is the estimated result tuple count for the indexscan.  This
 * is the same as path.parent->rows for a simple indexscan, but it is
 * different for a nestloop inner scan, because the additional indexquals
 * coming from join clauses make the scan more selective than the parent
 * rel's restrict clauses alone would do.
 *----------
 */
typedef struct IndexPath
{
        Path            path;
        IndexOptInfo *indexinfo;
        List       *indexclauses;
        List       *indexquals;
        bool            isjoininner;
        ScanDirection indexscandir;
        Cost            indextotalcost;
        Selectivity indexselectivity;
        double          rows;                   /* estimated number of result tuples */
} IndexPath;

/*
 * BitmapHeapPath represents one or more indexscans that generate TID bitmaps
 * instead of directly accessing the heap, followed by AND/OR combinations
 * to produce a single bitmap, followed by a heap scan that uses the bitmap.
 * Note that the output is always considered unordered, since it will come
 * out in physical heap order no matter what the underlying indexes did.
 *
 * The individual indexscans are represented by IndexPath nodes, and any
 * logic on top of them is represented by a tree of BitmapAndPath and
 * BitmapOrPath nodes.  Notice that we can use the same IndexPath node both
 * to represent a regular IndexScan plan, and as the child of a BitmapHeapPath
 * that represents scanning the same index using a BitmapIndexScan.  The
 * startup_cost and total_cost figures of an IndexPath always represent the
 * costs to use it as a regular IndexScan.      The costs of a BitmapIndexScan
 * can be computed using the IndexPath's indextotalcost and indexselectivity.
 *
 * BitmapHeapPaths can be nestloop inner indexscans.  The isjoininner and
 * rows fields serve the same purpose as for plain IndexPaths.
 */
typedef struct BitmapHeapPath
{
        Path            path;
        Path       *bitmapqual;         /* IndexPath, BitmapAndPath, BitmapOrPath */
        bool            isjoininner;    /* T if it's a nestloop inner scan */
        double          rows;                   /* estimated number of result tuples */
} BitmapHeapPath;

/*
 * BitmapAndPath represents a BitmapAnd plan node; it can only appear as
 * part of the substructure of a BitmapHeapPath.  The Path structure is
 * a bit more heavyweight than we really need for this, but for simplicity
 * we make it a derivative of Path anyway.
 */
typedef struct BitmapAndPath
{
        Path            path;
        List       *bitmapquals;        /* IndexPaths and BitmapOrPaths */
        Selectivity bitmapselectivity;
} BitmapAndPath;

/*
 * BitmapOrPath represents a BitmapOr plan node; it can only appear as
 * part of the substructure of a BitmapHeapPath.  The Path structure is
 * a bit more heavyweight than we really need for this, but for simplicity
 * we make it a derivative of Path anyway.
 */
typedef struct BitmapOrPath
{
        Path            path;
        List       *bitmapquals;        /* IndexPaths and BitmapAndPaths */
        Selectivity bitmapselectivity;
} BitmapOrPath;

/*
 * TidPath represents a scan by TID
 *
 * tidquals is an implicitly OR'ed list of qual expressions of the form
 * "CTID = pseudoconstant" or "CTID = ANY(pseudoconstant_array)".
 * Note they are bare expressions, not RestrictInfos.
 */
typedef struct TidPath
{
        Path            path;
        List       *tidquals;           /* qual(s) involving CTID = something */
} TidPath;

/*
 * AppendPath represents an Append plan, ie, successive execution of
 * several member plans.
 *
 * Note: it is possible for "subpaths" to contain only one, or even no,
 * elements.  These cases are optimized during create_append_plan.
 */
typedef struct AppendPath
{
        Path            path;
        List       *subpaths;           /* list of component Paths */
} AppendPath;

/*
 * ResultPath represents use of a Result plan node to compute a variable-free
 * targetlist with no underlying tables (a "SELECT expressions" query).
 * The query could have a WHERE clause, too, represented by "quals".
 *
 * Note that quals is a list of bare clauses, not RestrictInfos.
 */
typedef struct ResultPath
{
        Path            path;
        List       *quals;
} ResultPath;

/*
 * MaterialPath represents use of a Material plan node, i.e., caching of
 * the output of its subpath.  This is used when the subpath is expensive
 * and needs to be scanned repeatedly, or when we need mark/restore ability
 * and the subpath doesn't have it.
 */
typedef struct MaterialPath
{
        Path            path;
        Path       *subpath;
} MaterialPath;

/*
 * UniquePath represents elimination of distinct rows from the output of
 * its subpath.
 *
 * This is unlike the other Path nodes in that it can actually generate
 * different plans: either hash-based or sort-based implementation, or a
 * no-op if the input path can be proven distinct already.      The decision
 * is sufficiently localized that it's not worth having separate Path node
 * types.  (Note: in the no-op case, we could eliminate the UniquePath node
 * entirely and just return the subpath; but it's convenient to have a
 * UniquePath in the path tree to signal upper-level routines that the input
 * is known distinct.)
 */
typedef enum
{
        UNIQUE_PATH_NOOP,                       /* input is known unique already */
        UNIQUE_PATH_HASH,                       /* use hashing */
        UNIQUE_PATH_SORT                        /* use sorting */
} UniquePathMethod;

typedef struct UniquePath
{
        Path            path;
        Path       *subpath;
        UniquePathMethod umethod;
        double          rows;                   /* estimated number of result tuples */
} UniquePath;

/*
 * All join-type paths share these fields.
 */

typedef struct JoinPath
{
        Path            path;

        JoinType        jointype;

        Path       *outerjoinpath;      /* path for the outer side of the join */
        Path       *innerjoinpath;      /* path for the inner side of the join */

        List       *joinrestrictinfo;           /* RestrictInfos to apply to join */

        /*
         * See the notes for RelOptInfo to understand why joinrestrictinfo is
         * needed in JoinPath, and can't be merged into the parent RelOptInfo.
         */
} JoinPath;

/*
 * A nested-loop path needs no special fields.
 */

typedef JoinPath NestPath;

/*
 * A mergejoin path has these fields.
 *
 * path_mergeclauses lists the clauses (in the form of RestrictInfos)
 * that will be used in the merge.
 *
 * Note that the mergeclauses are a subset of the parent relation's
 * restriction-clause list.  Any join clauses that are not mergejoinable
 * appear only in the parent's restrict list, and must be checked by a
 * qpqual at execution time.
 *
 * outersortkeys (resp. innersortkeys) is NIL if the outer path
 * (resp. inner path) is already ordered appropriately for the
 * mergejoin.  If it is not NIL then it is a PathKeys list describing
 * the ordering that must be created by an explicit sort step.
 */

typedef struct MergePath
{
        JoinPath        jpath;
        List       *path_mergeclauses;          /* join clauses to be used for merge */
        List       *outersortkeys;      /* keys for explicit sort, if any */
        List       *innersortkeys;      /* keys for explicit sort, if any */
} MergePath;

/*
 * A hashjoin path has these fields.
 *
 * The remarks above for mergeclauses apply for hashclauses as well.
 *
 * Hashjoin does not care what order its inputs appear in, so we have
 * no need for sortkeys.
 */

typedef struct HashPath
{
        JoinPath        jpath;
        List       *path_hashclauses;           /* join clauses used for hashing */
} HashPath;

/*
 * Restriction clause info.
 *
 * We create one of these for each AND sub-clause of a restriction condition
 * (WHERE or JOIN/ON clause).  Since the restriction clauses are logically
 * ANDed, we can use any one of them or any subset of them to filter out
 * tuples, without having to evaluate the rest.  The RestrictInfo node itself
 * stores data used by the optimizer while choosing the best query plan.
 *
 * If a restriction clause references a single base relation, it will appear
 * in the baserestrictinfo list of the RelOptInfo for that base rel.
 *
 * If a restriction clause references more than one base rel, it will
 * appear in the joininfo list of every RelOptInfo that describes a strict
 * subset of the base rels mentioned in the clause.  The joininfo lists are
 * used to drive join tree building by selecting plausible join candidates.
 * The clause cannot actually be applied until we have built a join rel
 * containing all the base rels it references, however.
 *
 * When we construct a join rel that includes all the base rels referenced
 * in a multi-relation restriction clause, we place that clause into the
 * joinrestrictinfo lists of paths for the join rel, if neither left nor
 * right sub-path includes all base rels referenced in the clause.      The clause
 * will be applied at that join level, and will not propagate any further up
 * the join tree.  (Note: the "predicate migration" code was once intended to
 * push restriction clauses up and down the plan tree based on evaluation
 * costs, but it's dead code and is unlikely to be resurrected in the
 * foreseeable future.)
 *
 * Note that in the presence of more than two rels, a multi-rel restriction
 * might reach different heights in the join tree depending on the join
 * sequence we use.  So, these clauses cannot be associated directly with
 * the join RelOptInfo, but must be kept track of on a per-join-path basis.
 *
 * RestrictInfos that represent equivalence conditions (i.e., mergejoinable
 * equalities that are not outerjoin-delayed) are handled a bit differently.
 * Initially we attach them to the EquivalenceClasses that are derived from
 * them.  When we construct a scan or join path, we look through all the
 * EquivalenceClasses and generate derived RestrictInfos representing the
 * minimal set of conditions that need to be checked for this particular scan
 * or join to enforce that all members of each EquivalenceClass are in fact
 * equal in all rows emitted by the scan or join.
 *
 * When dealing with outer joins we have to be very careful about pushing qual
 * clauses up and down the tree.  An outer join's own JOIN/ON conditions must
 * be evaluated exactly at that join node, unless they are "degenerate"
 * conditions that reference only Vars from the nullable side of the join.
 * Quals appearing in WHERE or in a JOIN above the outer join cannot be pushed
 * down below the outer join, if they reference any nullable Vars.
 * RestrictInfo nodes contain a flag to indicate whether a qual has been
 * pushed down to a lower level than its original syntactic placement in the
 * join tree would suggest.  If an outer join prevents us from pushing a qual
 * down to its "natural" semantic level (the level associated with just the
 * base rels used in the qual) then we mark the qual with a "required_relids"
 * value including more than just the base rels it actually uses.  By
 * pretending that the qual references all the rels required to form the outer
 * join, we prevent it from being evaluated below the outer join's joinrel.
 * When we do form the outer join's joinrel, we still need to distinguish
 * those quals that are actually in that join's JOIN/ON condition from those
 * that appeared elsewhere in the tree and were pushed down to the join rel
 * because they used no other rels.  That's what the is_pushed_down flag is
 * for; it tells us that a qual is not an OUTER JOIN qual for the set of base
 * rels listed in required_relids.      A clause that originally came from WHERE
 * or an INNER JOIN condition will *always* have its is_pushed_down flag set.
 * It's possible for an OUTER JOIN clause to be marked is_pushed_down too,
 * if we decide that it can be pushed down into the nullable side of the join.
 * In that case it acts as a plain filter qual for wherever it gets evaluated.
 * (In short, is_pushed_down is only false for non-degenerate outer join
 * conditions.  Possibly we should rename it to reflect that meaning?)
 *
 * RestrictInfo nodes also contain an outerjoin_delayed flag, which is true
 * if the clause's applicability must be delayed due to any outer joins
 * appearing below its own syntactic level (ie, it references any Vars from
 * the nullable side of any lower outer join).
 *
 * In general, the referenced clause might be arbitrarily complex.      The
 * kinds of clauses we can handle as indexscan quals, mergejoin clauses,
 * or hashjoin clauses are limited (e.g., no volatile functions).  The code
 * for each kind of path is responsible for identifying the restrict clauses
 * it can use and ignoring the rest.  Clauses not implemented by an indexscan,
 * mergejoin, or hashjoin will be placed in the plan qual or joinqual field
 * of the finished Plan node, where they will be enforced by general-purpose
 * qual-expression-evaluation code.  (But we are still entitled to count
 * their selectivity when estimating the result tuple count, if we
 * can guess what it is...)
 *
 * When the referenced clause is an OR clause, we generate a modified copy
 * in which additional RestrictInfo nodes are inserted below the top-level
 * OR/AND structure.  This is a convenience for OR indexscan processing:
 * indexquals taken from either the top level or an OR subclause will have
 * associated RestrictInfo nodes.
 *
 * The can_join flag is set true if the clause looks potentially useful as
 * a merge or hash join clause, that is if it is a binary opclause with
 * nonoverlapping sets of relids referenced in the left and right sides.
 * (Whether the operator is actually merge or hash joinable isn't checked,
 * however.)
 *
 * The pseudoconstant flag is set true if the clause contains no Vars of
 * the current query level and no volatile functions.  Such a clause can be
 * pulled out and used as a one-time qual in a gating Result node.      We keep
 * pseudoconstant clauses in the same lists as other RestrictInfos so that
 * the regular clause-pushing machinery can assign them to the correct join
 * level, but they need to be treated specially for cost and selectivity
 * estimates.  Note that a pseudoconstant clause can never be an indexqual
 * or merge or hash join clause, so it's of no interest to large parts of
 * the planner.
 *
 * When join clauses are generated from EquivalenceClasses, there may be
 * several equally valid ways to enforce join equivalence, of which we need
 * apply only one.      We mark clauses of this kind by setting parent_ec to
 * point to the generating EquivalenceClass.  Multiple clauses with the same
 * parent_ec in the same join are redundant.
 */

typedef struct RestrictInfo
{
        NodeTag         type;

        Expr       *clause;                     /* the represented clause of WHERE or JOIN */

        bool            is_pushed_down; /* TRUE if clause was pushed down in level */

        bool            outerjoin_delayed;      /* TRUE if delayed by lower outer join */

        bool            can_join;               /* see comment above */

        bool            pseudoconstant; /* see comment above */

        /* The set of relids (varnos) actually referenced in the clause: */
        Relids          clause_relids;

        /* The set of relids required to evaluate the clause: */
        Relids          required_relids;

        /* These fields are set for any binary opclause: */
        Relids          left_relids;    /* relids in left side of clause */
        Relids          right_relids;   /* relids in right side of clause */

        /* This field is NULL unless clause is an OR clause: */
        Expr       *orclause;           /* modified clause with RestrictInfos */

        /* This field is NULL unless clause is potentially redundant: */
        EquivalenceClass *parent_ec;    /* generating EquivalenceClass */

        /* cache space for cost and selectivity */
        QualCost        eval_cost;              /* eval cost of clause; -1 if not yet set */
        Selectivity this_selec;         /* selectivity; -1 if not yet set */

        /* valid if clause is mergejoinable, else NIL */
        List       *mergeopfamilies;    /* opfamilies containing clause operator */

        /* cache space for mergeclause processing; NULL if not yet set */
        EquivalenceClass *left_ec;      /* EquivalenceClass containing lefthand */
        EquivalenceClass *right_ec; /* EquivalenceClass containing righthand */
        EquivalenceMember *left_em; /* EquivalenceMember for lefthand */
        EquivalenceMember *right_em;    /* EquivalenceMember for righthand */
        List       *scansel_cache;      /* list of MergeScanSelCache structs */

        /* transient workspace for use while considering a specific join path */
        bool            outer_is_left;  /* T = outer var on left, F = on right */

        /* valid if clause is hashjoinable, else InvalidOid: */
        Oid                     hashjoinoperator;               /* copy of clause operator */

        /* cache space for hashclause processing; -1 if not yet set */
        Selectivity left_bucketsize;    /* avg bucketsize of left side */
        Selectivity right_bucketsize;           /* avg bucketsize of right side */
} RestrictInfo;

/*
 * Since mergejoinscansel() is a relatively expensive function, and would
 * otherwise be invoked many times while planning a large join tree,
 * we go out of our way to cache its results.  Each mergejoinable
 * RestrictInfo carries a list of the specific sort orderings that have
 * been considered for use with it, and the resulting selectivities.
 */
typedef struct MergeScanSelCache
{
        /* Ordering details (cache lookup key) */
        Oid                     opfamily;               /* btree opfamily defining the ordering */
        int                     strategy;               /* sort direction (ASC or DESC) */
        bool            nulls_first;    /* do NULLs come before normal values? */
        /* Results */
        Selectivity leftstartsel;       /* first-join fraction for clause left side */
        Selectivity leftendsel;         /* last-join fraction for clause left side */
        Selectivity rightstartsel;      /* first-join fraction for clause right side */
        Selectivity rightendsel;        /* last-join fraction for clause right side */
} MergeScanSelCache;

/*
 * Inner indexscan info.
 *
 * An inner indexscan is one that uses one or more joinclauses as index
 * conditions (perhaps in addition to plain restriction clauses).  So it
 * can only be used as the inner path of a nestloop join where the outer
 * relation includes all other relids appearing in those joinclauses.
 * The set of usable joinclauses, and thus the best inner indexscan,
 * thus varies depending on which outer relation we consider; so we have
 * to recompute the best such paths for every join.  To avoid lots of
 * redundant computation, we cache the results of such searches.  For
 * each relation we compute the set of possible otherrelids (all relids
 * appearing in joinquals that could become indexquals for this table).
 * Two outer relations whose relids have the same intersection with this
 * set will have the same set of available joinclauses and thus the same
 * best inner indexscans for the inner relation.  By taking the intersection
 * before scanning the cache, we avoid recomputing when considering
 * join rels that differ only by the inclusion of irrelevant other rels.
 *
 * The search key also includes a bool showing whether the join being
 * considered is an outer join.  Since we constrain the join order for
 * outer joins, I believe that this bool can only have one possible value
 * for any particular lookup key; but store it anyway to avoid confusion.
 */

typedef struct InnerIndexscanInfo
{
        NodeTag         type;
        /* The lookup key: */
        Relids          other_relids;   /* a set of relevant other relids */
        bool            isouterjoin;    /* true if join is outer */
        /* Best paths for this lookup key (NULL if no available indexscans): */
        Path       *cheapest_startup_innerpath;         /* cheapest startup cost */
        Path       *cheapest_total_innerpath;           /* cheapest total cost */
} InnerIndexscanInfo;

/*
 * Outer join info.
 *
 * One-sided outer joins constrain the order of joining partially but not
 * completely.  We flatten such joins into the planner's top-level list of
 * relations to join, but record information about each outer join in an
 * OuterJoinInfo struct.  These structs are kept in the PlannerInfo node's
 * oj_info_list.
 *
 * min_lefthand and min_righthand are the sets of base relids that must be
 * available on each side when performing the outer join.  lhs_strict is
 * true if the outer join's condition cannot succeed when the LHS variables
 * are all NULL (this means that the outer join can commute with upper-level
 * outer joins even if it appears in their RHS).  We don't bother to set
 * lhs_strict for FULL JOINs, however.
 *
 * It is not valid for either min_lefthand or min_righthand to be empty sets;
 * if they were, this would break the logic that enforces join order.
 *
 * syn_lefthand and syn_righthand are the sets of base relids that are
 * syntactically below this outer join.  (These are needed to help compute
 * min_lefthand and min_righthand for higher joins, but are not used
 * thereafter.)
 *
 * delay_upper_joins is set TRUE if we detect a pushed-down clause that has
 * to be evaluated after this join is formed (because it references the RHS).
 * Any outer joins that have such a clause and this join in their RHS cannot
 * commute with this join, because that would leave noplace to check the
 * pushed-down clause.  (We don't track this for FULL JOINs, either.)
 *
 * Note: OuterJoinInfo directly represents only LEFT JOIN and FULL JOIN;
 * RIGHT JOIN is handled by switching the inputs to make it a LEFT JOIN.
 * We make an OuterJoinInfo for FULL JOINs even though there is no flexibility
 * of planning for them, because this simplifies make_join_rel()'s API.
 */

typedef struct OuterJoinInfo
{
        NodeTag         type;
        Relids          min_lefthand;   /* base relids in minimum LHS for join */
        Relids          min_righthand;  /* base relids in minimum RHS for join */
        Relids          syn_lefthand;   /* base relids syntactically within LHS */
        Relids          syn_righthand;  /* base relids syntactically within RHS */
        bool            is_full_join;   /* it's a FULL OUTER JOIN */
        bool            lhs_strict;             /* joinclause is strict for some LHS rel */
        bool            delay_upper_joins;              /* can't commute with upper RHS */
} OuterJoinInfo;

/*
 * IN clause info.
 *
 * When we convert top-level IN quals into join operations, we must restrict
 * the order of joining and use special join methods at some join points.
 * We record information about each such IN clause in an InClauseInfo struct.
 * These structs are kept in the PlannerInfo node's in_info_list.
 *
 * Note: sub_targetlist is just a list of Vars or expressions; it does not
 * contain TargetEntry nodes.
 */

typedef struct InClauseInfo
{
        NodeTag         type;
        Relids          lefthand;               /* base relids in lefthand expressions */
        Relids          righthand;              /* base relids coming from the subselect */
        List       *sub_targetlist; /* targetlist of original RHS subquery */
        List       *in_operators;       /* OIDs of the IN's equality operator(s) */
} InClauseInfo;

/*
 * Append-relation info.
 *
 * When we expand an inheritable table or a UNION-ALL subselect into an
 * "append relation" (essentially, a list of child RTEs), we build an
 * AppendRelInfo for each child RTE.  The list of AppendRelInfos indicates
 * which child RTEs must be included when expanding the parent, and each
 * node carries information needed to translate Vars referencing the parent
 * into Vars referencing that child.
 *
 * These structs are kept in the PlannerInfo node's append_rel_list.
 * Note that we just throw all the structs into one list, and scan the
 * whole list when desiring to expand any one parent.  We could have used
 * a more complex data structure (eg, one list per parent), but this would
 * be harder to update during operations such as pulling up subqueries,
 * and not really any easier to scan.  Considering that typical queries
 * will not have many different append parents, it doesn't seem worthwhile
 * to complicate things.
 *
 * Note: after completion of the planner prep phase, any given RTE is an
 * append parent having entries in append_rel_list if and only if its
 * "inh" flag is set.  We clear "inh" for plain tables that turn out not
 * to have inheritance children, and (in an abuse of the original meaning
 * of the flag) we set "inh" for subquery RTEs that turn out to be
 * flattenable UNION ALL queries.  This lets us avoid useless searches
 * of append_rel_list.
 *
 * Note: the data structure assumes that append-rel members are single
 * baserels.  This is OK for inheritance, but it prevents us from pulling
 * up a UNION ALL member subquery if it contains a join.  While that could
 * be fixed with a more complex data structure, at present there's not much
 * point because no improvement in the plan could result.
 */

typedef struct AppendRelInfo
{
        NodeTag         type;

        /*
         * These fields uniquely identify this append relationship.  There can be
         * (in fact, always should be) multiple AppendRelInfos for the same
         * parent_relid, but never more than one per child_relid, since a given
         * RTE cannot be a child of more than one append parent.
         */
        Index           parent_relid;   /* RT index of append parent rel */
        Index           child_relid;    /* RT index of append child rel */

        /*
         * For an inheritance appendrel, the parent and child are both regular
         * relations, and we store their rowtype OIDs here for use in translating
         * whole-row Vars.      For a UNION-ALL appendrel, the parent and child are
         * both subqueries with no named rowtype, and we store InvalidOid here.
         */
        Oid                     parent_reltype; /* OID of parent's composite type */
        Oid                     child_reltype;  /* OID of child's composite type */

        /*
         * The N'th element of this list is the integer column number of the child
         * column corresponding to the N'th column of the parent. A list element
         * is zero if it corresponds to a dropped column of the parent (this is
         * only possible for inheritance cases, not UNION ALL).
         */
        List       *col_mappings;       /* list of child attribute numbers */

        /*
         * The N'th element of this list is a Var or expression representing the
         * child column corresponding to the N'th column of the parent. This is
         * used to translate Vars referencing the parent rel into references to
         * the child.  A list element is NULL if it corresponds to a dropped
         * column of the parent (this is only possible for inheritance cases, not
         * UNION ALL).
         *
         * This might seem redundant with the col_mappings data, but it is handy
         * because flattening of sub-SELECTs that are members of a UNION ALL will
         * cause changes in the expressions that need to be substituted for a
         * parent Var.  Adjusting this data structure lets us track what really
         * needs to be substituted.
         *
         * Notice we only store entries for user columns (attno > 0).  Whole-row
         * Vars are special-cased, and system columns (attno < 0) need no special
         * translation since their attnos are the same for all tables.
         *
         * Caution: the Vars have varlevelsup = 0.      Be careful to adjust as needed
         * when copying into a subquery.
         */
        List       *translated_vars;    /* Expressions in the child's Vars */

        /*
         * We store the parent table's OID here for inheritance, or InvalidOid for
         * UNION ALL.  This is only needed to help in generating error messages if
         * an attempt is made to reference a dropped parent column.
         */
        Oid                     parent_reloid;  /* OID of parent relation */
} AppendRelInfo;

/*
 * glob->paramlist keeps track of the PARAM_EXEC slots that we have decided
 * we need for the query.  At runtime these slots are used to pass values
 * either down into subqueries (for outer references in subqueries) or up out
 * of subqueries (for the results of a subplan).  The n'th entry in the list
 * (n counts from 0) corresponds to Param->paramid = n.
 *
 * Each paramlist item shows the absolute query level it is associated with,
 * where the outermost query is level 1 and nested subqueries have higher
 * numbers.  The item the parameter slot represents can be one of three kinds:
 *
 * A Var: the slot represents a variable of that level that must be passed
 * down because subqueries have outer references to it.  The varlevelsup
 * value in the Var will always be zero.
 *
 * An Aggref (with an expression tree representing its argument): the slot
 * represents an aggregate expression that is an outer reference for some
 * subquery.  The Aggref itself has agglevelsup = 0, and its argument tree
 * is adjusted to match in level.
 *
 * A Param: the slot holds the result of a subplan (it is a setParam item
 * for that subplan).  The absolute level shown for such items corresponds
 * to the parent query of the subplan.
 *
 * Note: we detect duplicate Var parameters and coalesce them into one slot,
 * but we do not do this for Aggref or Param slots.
 */
typedef struct PlannerParamItem
{
        NodeTag         type;

        Node       *item;                       /* the Var, Aggref, or Param */
        Index           abslevel;               /* its absolute query level */
} PlannerParamItem;

#endif   /* RELATION_H */