From: Tom Lane Date: Fri, 27 Jun 2008 03:56:55 +0000 (+0000) Subject: Improve planner's estimation of the size of an append relation: rather than X-Git-Tag: REL8_4_BETA1~1232 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=2c2161a47d4765842868b019bb1814495863127c;p=postgresql Improve planner's estimation of the size of an append relation: rather than taking the maximum of any child rel's width, we should weight the widths proportionally to the number of rows expected from each child. In hindsight this is obviously correct because row width is really a proxy for the total physical size of the relation. Per discussion with Scott Carey (bug #4264). --- diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c index e6728f39fe..c833586813 100644 --- a/src/backend/optimizer/path/allpaths.c +++ b/src/backend/optimizer/path/allpaths.c @@ -8,13 +8,15 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.170 2008/04/01 00:48:33 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.171 2008/06/27 03:56:55 tgl Exp $ * *------------------------------------------------------------------------- */ #include "postgres.h" +#include + #ifdef OPTIMIZER_DEBUG #include "nodes/print.h" #endif @@ -263,6 +265,10 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, { int parentRTindex = rti; List *subpaths = NIL; + double parent_rows; + double parent_size; + double *parent_attrsizes; + int nattrs; ListCell *l; /* @@ -277,10 +283,23 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, errmsg("SELECT FOR UPDATE/SHARE is not supported for inheritance queries"))); /* - * Initialize to compute size estimates for whole append relation + * Initialize to compute size estimates for whole append relation. + * + * We handle width estimates by weighting the widths of different + * child rels proportionally to their number of rows. This is sensible + * because the use of width estimates is mainly to compute the total + * relation "footprint" if we have to sort or hash it. To do this, + * we sum the total equivalent size (in "double" arithmetic) and then + * divide by the total rowcount estimate. This is done separately for + * the total rel width and each attribute. + * + * Note: if you consider changing this logic, beware that child rels could + * have zero rows and/or width, if they were excluded by constraints. */ - rel->rows = 0; - rel->width = 0; + parent_rows = 0; + parent_size = 0; + nattrs = rel->max_attr - rel->min_attr + 1; + parent_attrsizes = (double *) palloc0(nattrs * sizeof(double)); /* * Generate access paths for each member relation, and pick the cheapest @@ -379,38 +398,53 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, subpaths = lappend(subpaths, childpath); /* - * Propagate size information from the child back to the parent. For - * simplicity, we use the largest widths from any child as the parent - * estimates. (If you want to change this, beware of child - * attr_widths[] entries that haven't been set and are still 0.) + * Accumulate size information from each child. */ - rel->rows += childrel->rows; - if (childrel->width > rel->width) - rel->width = childrel->width; - - forboth(parentvars, rel->reltargetlist, - childvars, childrel->reltargetlist) + if (childrel->rows > 0) { - Var *parentvar = (Var *) lfirst(parentvars); - Var *childvar = (Var *) lfirst(childvars); + parent_rows += childrel->rows; + parent_size += childrel->width * childrel->rows; - if (IsA(parentvar, Var) && - IsA(childvar, Var)) + forboth(parentvars, rel->reltargetlist, + childvars, childrel->reltargetlist) { - int pndx = parentvar->varattno - rel->min_attr; - int cndx = childvar->varattno - childrel->min_attr; + Var *parentvar = (Var *) lfirst(parentvars); + Var *childvar = (Var *) lfirst(childvars); - if (childrel->attr_widths[cndx] > rel->attr_widths[pndx]) - rel->attr_widths[pndx] = childrel->attr_widths[cndx]; + if (IsA(parentvar, Var) && + IsA(childvar, Var)) + { + int pndx = parentvar->varattno - rel->min_attr; + int cndx = childvar->varattno - childrel->min_attr; + + parent_attrsizes[pndx] += childrel->attr_widths[cndx] * childrel->rows; + } } } } + /* + * Save the finished size estimates. + */ + rel->rows = parent_rows; + if (parent_rows > 0) + { + int i; + + rel->width = rint(parent_size / parent_rows); + for (i = 0; i < nattrs; i++) + rel->attr_widths[i] = rint(parent_attrsizes[i] / parent_rows); + } + else + rel->width = 0; /* attr_widths should be zero already */ + /* * Set "raw tuples" count equal to "rows" for the appendrel; needed * because some places assume rel->tuples is valid for any baserel. */ - rel->tuples = rel->rows; + rel->tuples = parent_rows; + + pfree(parent_attrsizes); /* * Finally, build Append path and install it as the only access path for diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c index 109a014547..0884cda607 100644 --- a/src/backend/optimizer/plan/createplan.c +++ b/src/backend/optimizer/plan/createplan.c @@ -10,13 +10,14 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.240 2008/04/17 21:22:14 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.241 2008/06/27 03:56:55 tgl Exp $ * *------------------------------------------------------------------------- */ #include "postgres.h" #include +#include #include "access/skey.h" #include "nodes/makefuncs.h" @@ -2312,6 +2313,7 @@ make_append(List *appendplans, bool isTarget, List *tlist) { Append *node = makeNode(Append); Plan *plan = &node->plan; + double total_size; ListCell *subnode; /* @@ -2322,7 +2324,7 @@ make_append(List *appendplans, bool isTarget, List *tlist) plan->startup_cost = 0; plan->total_cost = 0; plan->plan_rows = 0; - plan->plan_width = 0; + total_size = 0; foreach(subnode, appendplans) { Plan *subplan = (Plan *) lfirst(subnode); @@ -2331,9 +2333,12 @@ make_append(List *appendplans, bool isTarget, List *tlist) plan->startup_cost = subplan->startup_cost; plan->total_cost += subplan->total_cost; plan->plan_rows += subplan->plan_rows; - if (plan->plan_width < subplan->plan_width) - plan->plan_width = subplan->plan_width; + total_size += subplan->plan_width * subplan->plan_rows; } + if (plan->plan_rows > 0) + plan->plan_width = rint(total_size / plan->plan_rows); + else + plan->plan_width = 0; plan->targetlist = tlist; plan->qual = NIL;