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Revise the planner's handling of "pseudoconstant" WHERE clauses, that is

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clauses containing no variables and no volatile functions.  Such a clause
can be used as a one-time qual in a gating Result plan node, to suppress
plan execution entirely when it is false.  Even when the clause is true,
putting it in a gating node wins by avoiding repeated evaluation of the
clause.  In previous PG releases, query_planner() would do this for
pseudoconstant clauses appearing at the top level of the jointree, but
there was no ability to generate a gating Result deeper in the plan tree.
To fix it, get rid of the special case in query_planner(), and instead
process pseudoconstant clauses through the normal RestrictInfo qual
distribution mechanism.  When a pseudoconstant clause is found attached to
a path node in create_plan(), pull it out and generate a gating Result at
that point.  This requires special-casing pseudoconstants in selectivity
estimation and cost_qual_eval, but on the whole it's pretty clean.
It probably even makes the planner a bit faster than before for the normal
case of no pseudoconstants, since removing pull_constant_clauses saves one
useless traversal of the qual tree.  Per gripe from Phil Frost.
This commit is contained in:
Tom Lane
2006-07-01 18:38:33 +00:00
parent 68628fc38e
commit cffd89ca73
20 changed files with 464 additions and 294 deletions
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10
src/backend/optimizer/path/allpaths.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.146 2006/05/02 04:34:18 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.147 2006/07/01 18:38:32 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -446,7 +446,9 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
* There are several cases where we cannot push down clauses. Restrictions
* involving the subquery are checked by subquery_is_pushdown_safe().
* Restrictions on individual clauses are checked by
* qual_is_pushdown_safe().
* qual_is_pushdown_safe(). Also, we don't want to push down
* pseudoconstant clauses; better to have the gating node above the
* subquery.
*
* Non-pushed-down clauses will get evaluated as qpquals of the
* SubqueryScan node.
@ -466,7 +468,8 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);
Node *clause = (Node *) rinfo->clause;
if (qual_is_pushdown_safe(subquery, rti, clause, differentTypes))
if (!rinfo->pseudoconstant &&
qual_is_pushdown_safe(subquery, rti, clause, differentTypes))
{
/* Push it down */
subquery_push_qual(subquery, rte, rti, clause);
@ -1066,7 +1069,6 @@ print_path(PlannerInfo *root, Path *path, int indent)
break;
case T_ResultPath:
ptype = "Result";
subpath = ((ResultPath *) path)->subpath;
break;
case T_MaterialPath:
ptype = "Material";