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Allow user control of CTE materialization, and change the default behavior.

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Historically we've always materialized the full output of a CTE query,
treating WITH as an optimization fence (so that, for example, restrictions
from the outer query cannot be pushed into it).  This is appropriate when
the CTE query is INSERT/UPDATE/DELETE, or is recursive; but when the CTE
query is non-recursive and side-effect-free, there's no hazard of changing
the query results by pushing restrictions down.

Another argument for materialization is that it can avoid duplicate
computation of an expensive WITH query --- but that only applies if
the WITH query is called more than once in the outer query.  Even then
it could still be a net loss, if each call has restrictions that
would allow just a small part of the WITH query to be computed.

Hence, let's change the behavior for WITH queries that are non-recursive
and side-effect-free.  By default, we will inline them into the outer
query (removing the optimization fence) if they are called just once.
If they are called more than once, we will keep the old behavior by
default, but the user can override this and force inlining by specifying
NOT MATERIALIZED.  Lastly, the user can force the old behavior by
specifying MATERIALIZED; this would mainly be useful when the query had
deliberately been employing WITH as an optimization fence to prevent a
poor choice of plan.

Andreas Karlsson, Andrew Gierth, David Fetter

Discussion: https://postgr.es/m/87sh48ffhb.fsf@news-spur.riddles.org.uk
This commit is contained in:
Tom Lane
2019-02-16 16:11:12 -05:00
parent 79730e2a9b
commit 608b167f9f
23 changed files with 580 additions and 44 deletions
Download Patch File Download Diff File Expand all files Collapse all files

14
src/backend/parser/gram.y
View File

@ -479,7 +479,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
%type <list> row explicit_row implicit_row type_list array_expr_list
%type <node> case_expr case_arg when_clause case_default
%type <list> when_clause_list
%type <ival> sub_type
%type <ival> sub_type opt_materialized
%type <value> NumericOnly
%type <list> NumericOnly_list
%type <alias> alias_clause opt_alias_clause
@ -11344,17 +11344,24 @@ cte_list:
| cte_list ',' common_table_expr { $$ = lappend($1, $3); }
;
common_table_expr: name opt_name_list AS '(' PreparableStmt ')'
common_table_expr: name opt_name_list AS opt_materialized '(' PreparableStmt ')'
{
CommonTableExpr *n = makeNode(CommonTableExpr);
n->ctename = $1;
n->aliascolnames = $2;
n->ctequery = $5;
n->ctematerialized = $4;
n->ctequery = $6;
n->location = @1;
$$ = (Node *) n;
}
;
opt_materialized:
MATERIALIZED { $$ = CTEMaterializeAlways; }
| NOT MATERIALIZED { $$ = CTEMaterializeNever; }
| /*EMPTY*/ { $$ = CTEMaterializeDefault; }
;
opt_with_clause:
with_clause { $$ = $1; }
| /*EMPTY*/ { $$ = NULL; }
@ -16237,6 +16244,7 @@ makeRecursiveViewSelect(char *relname, List *aliases, Node *query)
/* create common table expression */
cte->ctename = relname;
cte->aliascolnames = aliases;
cte->ctematerialized = CTEMaterializeDefault;
cte->ctequery = query;
cte->location = -1;