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Rearrange the querytree representation of ORDER BY/GROUP BY/DISTINCT items

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as per my recent proposal:

1. Fold SortClause and GroupClause into a single node type SortGroupClause.
We were already relying on them to be struct-equivalent, so using two node
tags wasn't accomplishing much except to get in the way of comparing items
with equal().

2. Add an "eqop" field to SortGroupClause to carry the associated equality
operator.  This is cheap for the parser to get at the same time it's looking
up the sort operator, and storing it eliminates the need for repeated
not-so-cheap lookups during planning.  In future this will also let us
represent GROUP/DISTINCT operations on datatypes that have hash opclasses
but no btree opclasses (ie, they have equality but no natural sort order).
The previous representation simply didn't work for that, since its only
indicator of comparison semantics was a sort operator.

3. Add a hasDistinctOn boolean to struct Query to explicitly record whether
the distinctClause came from DISTINCT or DISTINCT ON.  This allows removing
some complicated and not 100% bulletproof code that attempted to figure
that out from the distinctClause alone.

This patch doesn't in itself create any new capability, but it's necessary
infrastructure for future attempts to use hash-based grouping for DISTINCT
and UNION/INTERSECT/EXCEPT.
This commit is contained in:
Tom Lane
2008-08-02 21:32:01 +00:00
parent 49f001d81e
commit 9511304752
33 changed files with 764 additions and 857 deletions
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17
src/backend/optimizer/path/allpaths.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.171 2008/06/27 03:56:55 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.172 2008/08/02 21:31:59 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -963,11 +963,12 @@ compare_tlist_datatypes(List *tlist, List *colTypes,
*
* 4. If the subquery uses DISTINCT ON, we must not push down any quals that
* refer to non-DISTINCT output columns, because that could change the set
* of rows returned. This condition is vacuous for DISTINCT, because then
* there are no non-DISTINCT output columns, but unfortunately it's fairly
* expensive to tell the difference between DISTINCT and DISTINCT ON in the
* parsetree representation. It's cheaper to just make sure all the Vars
* in the qual refer to DISTINCT columns.
* of rows returned. (This condition is vacuous for DISTINCT, because then
* there are no non-DISTINCT output columns, so we needn't check. But note
* we are assuming that the qual can't distinguish values that the DISTINCT
* operator sees as equal. This is a bit shaky but we have no way to test
* for the case, and it's unlikely enough that we shouldn't refuse the
* optimization just because it could theoretically happen.)
*
* 5. We must not push down any quals that refer to subselect outputs that
* return sets, else we'd introduce functions-returning-sets into the
@ -1030,8 +1031,8 @@ qual_is_pushdown_safe(Query *subquery, Index rti, Node *qual,
Assert(tle != NULL);
Assert(!tle->resjunk);
/* If subquery uses DISTINCT or DISTINCT ON, check point 4 */
if (subquery->distinctClause != NIL &&
/* If subquery uses DISTINCT ON, check point 4 */
if (subquery->hasDistinctOn &&
!targetIsInSortList(tle, InvalidOid, subquery->distinctClause))
{
/* non-DISTINCT column, so fail */