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Fix index-only scan plans when not all index columns can be returned.

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If an index has both returnable and non-returnable columns, and one of
the non-returnable columns is an expression using a Var that is in a
returnable column, then a query returning that expression could result
in an index-only scan plan that attempts to read the non-returnable
column, instead of recomputing the expression from the returnable
column as intended.

To fix, redefine the "indextlist" list of an IndexOnlyScan plan node
as containing null Consts in place of any non-returnable columns.
This solves the problem by preventing setrefs.c from falsely matching
to such entries.  The executor is happy since it only cares about the
exposed types of the entries, and ruleutils.c doesn't care because a
correct plan won't reference those entries.  I considered some other
ways to prevent setrefs.c from doing the wrong thing, but this way
seems good since (a) it allows a very localized fix, (b) it makes
the indextlist structure more compact in many cases, and (c) the
indextlist is now a more faithful representation of what the index AM
will actually produce, viz. nulls for any non-returnable columns.

This is easier to hit since we introduced included columns, but it's
possible to construct failing examples without that, as per the
added regression test.  Hence, back-patch to all supported branches.

Per bug #17350 from Louis Jachiet.

Discussion: https://postgr.es/m/17350-b5bdcf476e5badbb@postgresql.org
This commit is contained in:
Tom Lane
2022-01-01 16:12:03 -05:00
parent cb4f1be43a
commit cabea571d1
4 changed files with 92 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

53
src/backend/optimizer/plan/createplan.c
View File

@ -20,6 +20,7 @@
#include <math.h>
#include "access/sysattr.h"
#include "catalog/pg_am.h"
#include "catalog/pg_class.h"
#include "foreign/fdwapi.h"
#include "miscadmin.h"
@ -191,6 +192,7 @@ static IndexOnlyScan *make_indexonlyscan(List *qptlist, List *qpqual,
List *indexqual, List *indexorderby,
List *indextlist,
ScanDirection indexscandir);
static List *make_indexonly_tlist(IndexOptInfo *indexinfo);
static BitmapIndexScan *make_bitmap_indexscan(Index scanrelid, Oid indexid,
List *indexqual,
List *indexqualorig);
@ -621,7 +623,7 @@ create_scan_plan(PlannerInfo *root, Path *best_path, int flags)
if (best_path->pathtype == T_IndexOnlyScan)
{
/* For index-only scan, the preferred tlist is the index's */
tlist = copyObject(((IndexPath *) best_path)->indexinfo->indextlist);
tlist = copyObject(make_indexonly_tlist(((IndexPath *) best_path)->indexinfo));
/*
* Transfer sortgroupref data to the replacement tlist, if
@ -3070,7 +3072,7 @@ create_indexscan_plan(PlannerInfo *root,
indexoid,
fixed_indexquals,
fixed_indexorderbys,
best_path->indexinfo->indextlist,
make_indexonly_tlist(best_path->indexinfo),
best_path->indexscandir);
else
scan_plan = (Scan *) make_indexscan(tlist,
@ -5462,6 +5464,53 @@ make_indexonlyscan(List *qptlist,
return node;
}
/*
* make_indexonly_tlist
*
* Construct the indextlist for an IndexOnlyScan plan node.
* We must replace any column that can't be returned by the index AM
* with a null Const of the appropriate datatype. This is necessary
* to prevent setrefs.c from trying to use the value of such a column,
* and anyway it makes the indextlist a better representative of what
* the indexscan will really return. (We do this here, not where the
* IndexOptInfo is originally constructed, because earlier planner
* steps need to know what is in such columns.)
*/
static List *
make_indexonly_tlist(IndexOptInfo *indexinfo)
{
List *result;
int i;
ListCell *lc;
/* We needn't work hard for the common case of btrees. */
if (indexinfo->relam == BTREE_AM_OID)
return indexinfo->indextlist;
result = NIL;
i = 0;
foreach(lc, indexinfo->indextlist)
{
TargetEntry *indextle = (TargetEntry *) lfirst(lc);
if (indexinfo->canreturn[i])
result = lappend(result, indextle);
else
{
TargetEntry *newtle = makeNode(TargetEntry);
Node *texpr = (Node *) indextle->expr;
memcpy(newtle, indextle, sizeof(TargetEntry));
newtle->expr = (Expr *) makeNullConst(exprType(texpr),
exprTypmod(texpr),
exprCollation(texpr));
result = lappend(result, newtle);
}
i++;
}
return result;
}
static BitmapIndexScan *
make_bitmap_indexscan(Index scanrelid,
Oid indexid,