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Recognize that IN subqueries return already-unique results if they use 

UNION/INTERSECT/EXCEPT (without ALL).  This adds on to the previous
optimization for subqueries using DISTINCT.
This commit is contained in:
Tom Lane 2004-01-19 03:49:41 +00:00
parent 5c625a9326
commit 864412fd0a
1 changed files with 35 additions and 6 deletions
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41
src/backend/optimizer/util/pathnode.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/util/pathnode.c,v 1.99 2004/01/05 23:39:54 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/util/pathnode.c,v 1.100 2004/01/19 03:49:41 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -33,6 +33,7 @@
#include "utils/syscache.h"
static bool is_distinct_query(Query *query);
static bool hash_safe_tlist(List *tlist);
@ -553,16 +554,14 @@ create_unique_path(Query *root, RelOptInfo *rel, Path *subpath)
pathnode->subpath = subpath;
/*
* If the input is a subquery that uses DISTINCT, we don't need to do
* anything; its output is already unique. (Are there any other cases
* in which we can easily prove the input must be distinct?)
* If the input is a subquery whose output must be unique already,
* we don't need to do anything.
*/
if (rel->rtekind == RTE_SUBQUERY)
{
RangeTblEntry *rte = rt_fetch(rel->relid, root->rtable);
Query *subquery = rte->subquery;
if (has_distinct_clause(subquery))
if (is_distinct_query(rte->subquery))
{
pathnode->umethod = UNIQUE_PATH_NOOP;
pathnode->rows = rel->rows;
@ -667,6 +666,36 @@ create_unique_path(Query *root, RelOptInfo *rel, Path *subpath)
return pathnode;
}
/*
* is_distinct_query - does query never return duplicate rows?
*/
static bool
is_distinct_query(Query *query)
{
/* DISTINCT (but not DISTINCT ON) guarantees uniqueness */
if (has_distinct_clause(query))
return true;
/* UNION, INTERSECT, EXCEPT guarantee uniqueness, except with ALL */
if (query->setOperations)
{
SetOperationStmt *topop = (SetOperationStmt *) query->setOperations;
Assert(IsA(topop, SetOperationStmt));
Assert(topop->op != SETOP_NONE);
if (!topop->all)
return true;
}
/*
* XXX Are there any other cases in which we can easily see the result
* must be distinct?
*/
return false;
}
/*
* hash_safe_tlist - can datatypes of given tlist be hashed?
*