*
*
* IDENTIFICATION
- * $Header: /cvsroot/pgsql/src/backend/optimizer/prep/prepqual.c,v 1.21 2000/01/26 05:56:39 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/optimizer/prep/prepqual.c,v 1.22 2000/01/28 03:22:36 tgl Exp $
*
*-------------------------------------------------------------------------
*/
static Expr *and_normalize(List *andlist);
static Expr *qual_cleanup(Expr *qual);
static List *remove_duplicates(List *list);
-static int count_bool_nodes(Expr *qual);
+static void count_bool_nodes(Expr *qual, double *nodes,
+ double *cnfnodes, double *dnfnodes);
/*****************************************************************************
*
List *
canonicalize_qual(Expr *qual, bool removeAndFlag)
{
- Expr *newqual,
- *cnfqual,
- *dnfqual;
- int qualcnt,
- cnfcnt,
- dnfcnt;
+ Expr *newqual;
+ double nodes,
+ cnfnodes,
+ dnfnodes;
+ bool cnfok,
+ dnfok;
if (qual == NULL)
return NIL;
* This improvement is always worthwhile, so do it unconditionally.
*/
qual = flatten_andors(qual);
+
/* Push down NOTs. We do this only in the top-level boolean
* expression, without examining arguments of operators/functions.
* Even so, it might not be a win if we are unable to find negators
- * for all the operators involved; so we keep the flattened-but-not-
- * NOT-pushed qual as the reference point for comparsions.
+ * for all the operators involved; perhaps we should compare before-
+ * and-after tree sizes?
*/
newqual = find_nots(qual);
- /*
- * Generate both CNF and DNF forms from newqual.
- */
- /* Normalize into conjunctive normal form, and clean up the result. */
- cnfqual = qual_cleanup(find_ors(newqual));
- /* Likewise for DNF */
- dnfqual = qual_cleanup(find_ands(newqual));
/*
- * Now, choose whether to return qual, cnfqual, or dnfqual.
+ * Choose whether to convert to CNF, or DNF, or leave well enough alone.
*
- * First heuristic is to forget about either CNF or DNF if it shows
+ * We make an approximate estimate of the number of bottom-level nodes
+ * that will appear in the CNF and DNF forms of the query.
+ */
+ count_bool_nodes(newqual, &nodes, &cnfnodes, &dnfnodes);
+ /*
+ * First heuristic is to forget about *both* normal forms if there are
+ * a huge number of terms in the qual clause. This would only happen
+ * with machine-generated queries, presumably; and most likely such
+ * a query is already in either CNF or DNF.
+ */
+ cnfok = dnfok = true;
+ if (nodes >= 500.0)
+ cnfok = dnfok = false;
+ /*
+ * Second heuristic is to forget about either CNF or DNF if it shows
* unreasonable growth compared to the original form of the qual,
* where we define "unreasonable" a tad arbitrarily as 4x more
* operators.
*/
- qualcnt = count_bool_nodes(qual);
- cnfcnt = count_bool_nodes(cnfqual);
- dnfcnt = count_bool_nodes(dnfqual);
- if (cnfcnt >= 4 * qualcnt)
- cnfqual = NULL; /* mark CNF not usable */
- if (dnfcnt >= 4 * qualcnt)
- dnfqual = NULL; /* mark DNF not usable */
-
+ if (cnfnodes >= 4.0 * nodes)
+ cnfok = false;
+ if (dnfnodes >= 4.0 * nodes)
+ dnfok = false;
/*
- * Second heuristic is to prefer DNF if only one relation is mentioned
- * and it is smaller than the CNF representation.
+ * Third heuristic is to prefer DNF if top level is already an OR,
+ * and only one relation is mentioned, and DNF is no larger than
+ * the CNF representation. (Pretty shaky; can we improve on this?)
*/
- if (dnfqual && dnfcnt < cnfcnt && NumRelids((Node *) dnfqual) == 1)
- cnfqual = NULL;
-
+ if (dnfok && dnfnodes <= cnfnodes && or_clause((Node *) newqual) &&
+ NumRelids((Node *) newqual) == 1)
+ cnfok = false;
/*
* Otherwise, we prefer CNF.
*
* XXX obviously, these rules could be improved upon.
*/
-
- /* pick preferred survivor */
- if (cnfqual)
- newqual = cnfqual;
- else if (dnfqual)
- newqual = dnfqual;
- else
- newqual = qual;
+ if (cnfok)
+ {
+ /* Normalize into conjunctive normal form, and clean up the result. */
+ newqual = qual_cleanup(find_ors(newqual));
+ }
+ else if (dnfok)
+ {
+ /* Normalize into disjunctive normal form, and clean up the result. */
+ newqual = qual_cleanup(find_ands(newqual));
+ }
/* Convert to implicit-AND list if requested */
if (removeAndFlag)
/*
* count_bool_nodes
* Support for heuristics in canonicalize_qual(): count the
- * number of nodes in the top level AND/OR/NOT part of a qual tree
+ * number of nodes that are inputs to the top level AND/OR/NOT
+ * part of a qual tree, and estimate how many nodes will appear
+ * in the CNF'ified or DNF'ified equivalent of the expression.
+ *
+ * This is just an approximate calculation; it doesn't deal with NOTs
+ * very well, and of course it cannot detect possible simplifications
+ * from eliminating duplicate subclauses. The idea is just to cheaply
+ * determine whether CNF will be markedly worse than DNF or vice versa.
+ *
+ * The counts/estimates are represented as doubles to avoid risk of overflow.
*/
-static int
-count_bool_nodes(Expr *qual)
+static void
+count_bool_nodes(Expr *qual,
+ double *nodes,
+ double *cnfnodes,
+ double *dnfnodes)
{
- if (qual == NULL)
- return 0;
+ List *temp;
+ double subnodes, subcnfnodes, subdnfnodes;
- if (and_clause((Node *) qual) ||
- or_clause((Node *) qual))
+ if (and_clause((Node *) qual))
{
- int sum = 1; /* for the and/or itself */
- List *temp;
+ *nodes = *cnfnodes = 0.0;
+ *dnfnodes = 1.0; /* DNF nodes will be product of sub-counts */
foreach(temp, qual->args)
- sum += count_bool_nodes(lfirst(temp));
+ {
+ count_bool_nodes(lfirst(temp),
+ &subnodes, &subcnfnodes, &subdnfnodes);
+ *nodes += subnodes;
+ *cnfnodes += subcnfnodes;
+ *dnfnodes *= subdnfnodes;
+ }
+ /* we could get dnfnodes < cnfnodes here, if all the sub-nodes are
+ * simple ones with count 1. Make sure dnfnodes isn't too small.
+ */
+ if (*dnfnodes < *cnfnodes)
+ *dnfnodes = *cnfnodes;
+ }
+ else if (or_clause((Node *) qual))
+ {
+ *nodes = *dnfnodes = 0.0;
+ *cnfnodes = 1.0; /* CNF nodes will be product of sub-counts */
- return sum;
+ foreach(temp, qual->args)
+ {
+ count_bool_nodes(lfirst(temp),
+ &subnodes, &subcnfnodes, &subdnfnodes);
+ *nodes += subnodes;
+ *cnfnodes *= subcnfnodes;
+ *dnfnodes += subdnfnodes;
+ }
+ /* we could get cnfnodes < dnfnodes here, if all the sub-nodes are
+ * simple ones with count 1. Make sure cnfnodes isn't too small.
+ */
+ if (*cnfnodes < *dnfnodes)
+ *cnfnodes = *dnfnodes;
}
else if (not_clause((Node *) qual))
- return count_bool_nodes(get_notclausearg(qual)) + 1;
+ {
+ count_bool_nodes(get_notclausearg(qual),
+ nodes, cnfnodes, dnfnodes);
+ }
else
- return 1; /* anything else counts 1 for my purposes */
+ {
+ /* anything else counts 1 for my purposes */
+ *nodes = *cnfnodes = *dnfnodes = 1.0;
+ }
}
projects / postgresql / commitdiff