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subqueries into the same thing you'd have gotten from IN (except always with unknownEqFalse = true, so as to get the proper semantics for an EXISTS). I believe this fixes the last case within CVS HEAD in which an EXISTS could give worse performance than an equivalent IN subquery. The tricky part of this is that if the upper query probes the EXISTS for only a few rows, the hashing implementation can actually be worse than the default, and therefore we need to make a cost-based decision about which way to use. But at the time when the planner generates plans for subqueries, it doesn't really know how many times the subquery will be executed. The least invasive solution seems to be to generate both plans and postpone the choice until execution. Therefore, in a query that has been optimized this way, EXPLAIN will show two subplans for the EXISTS, of which only one will actually get executed. There is a lot more that could be done based on this infrastructure: in particular it's interesting to consider switching to the hash plan if we start out using the non-hashed plan but find a lot more upper rows going by than we expected. I have therefore left some minor inefficiencies in place, such as initializing both subplans even though we will currently only use one.
108 lines
3.8 KiB
C
108 lines
3.8 KiB
C
/*-------------------------------------------------------------------------
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*
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* clauses.h
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* prototypes for clauses.c.
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*
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*
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* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* $PostgreSQL: pgsql/src/include/optimizer/clauses.h,v 1.93 2008/08/22 00:16:04 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef CLAUSES_H
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#define CLAUSES_H
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#include "nodes/relation.h"
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#define is_opclause(clause) ((clause) != NULL && IsA(clause, OpExpr))
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#define is_funcclause(clause) ((clause) != NULL && IsA(clause, FuncExpr))
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typedef struct
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{
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int numAggs; /* total number of aggregate calls */
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int numDistinctAggs; /* number that use DISTINCT */
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Size transitionSpace; /* for pass-by-ref transition data */
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} AggClauseCounts;
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extern Expr *make_opclause(Oid opno, Oid opresulttype, bool opretset,
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Expr *leftop, Expr *rightop);
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extern Node *get_leftop(Expr *clause);
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extern Node *get_rightop(Expr *clause);
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extern bool not_clause(Node *clause);
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extern Expr *make_notclause(Expr *notclause);
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extern Expr *get_notclausearg(Expr *notclause);
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extern bool or_clause(Node *clause);
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extern Expr *make_orclause(List *orclauses);
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extern bool and_clause(Node *clause);
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extern Expr *make_andclause(List *andclauses);
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extern Node *make_and_qual(Node *qual1, Node *qual2);
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extern Expr *make_ands_explicit(List *andclauses);
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extern List *make_ands_implicit(Expr *clause);
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extern bool contain_agg_clause(Node *clause);
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extern void count_agg_clauses(Node *clause, AggClauseCounts *counts);
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extern bool expression_returns_set(Node *clause);
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extern double expression_returns_set_rows(Node *clause);
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extern bool contain_subplans(Node *clause);
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extern bool contain_mutable_functions(Node *clause);
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extern bool contain_volatile_functions(Node *clause);
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extern bool contain_nonstrict_functions(Node *clause);
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extern Relids find_nonnullable_rels(Node *clause);
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extern List *find_nonnullable_vars(Node *clause);
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extern List *find_forced_null_vars(Node *clause);
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extern Var *find_forced_null_var(Node *clause);
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extern bool is_pseudo_constant_clause(Node *clause);
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extern bool is_pseudo_constant_clause_relids(Node *clause, Relids relids);
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extern int NumRelids(Node *clause);
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extern void CommuteOpExpr(OpExpr *clause);
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extern void CommuteRowCompareExpr(RowCompareExpr *clause);
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extern Node *strip_implicit_coercions(Node *node);
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extern void set_coercionform_dontcare(Node *node);
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extern Node *eval_const_expressions(PlannerInfo *root, Node *node);
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extern Node *estimate_expression_value(PlannerInfo *root, Node *node);
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extern Query *inline_set_returning_function(PlannerInfo *root, Node *node);
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extern bool expression_tree_walker(Node *node, bool (*walker) (),
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void *context);
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extern Node *expression_tree_mutator(Node *node, Node *(*mutator) (),
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void *context);
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/* flags bits for query_tree_walker and query_tree_mutator */
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#define QTW_IGNORE_RT_SUBQUERIES 0x01 /* subqueries in rtable */
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#define QTW_IGNORE_JOINALIASES 0x02 /* JOIN alias var lists */
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#define QTW_DONT_COPY_QUERY 0x04 /* do not copy top Query */
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extern bool query_tree_walker(Query *query, bool (*walker) (),
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void *context, int flags);
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extern Query *query_tree_mutator(Query *query, Node *(*mutator) (),
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void *context, int flags);
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extern bool range_table_walker(List *rtable, bool (*walker) (),
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void *context, int flags);
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extern List *range_table_mutator(List *rtable, Node *(*mutator) (),
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void *context, int flags);
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extern bool query_or_expression_tree_walker(Node *node, bool (*walker) (),
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void *context, int flags);
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extern Node *query_or_expression_tree_mutator(Node *node, Node *(*mutator) (),
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void *context, int flags);
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#endif /* CLAUSES_H */
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