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Fix code for re-finding scan position in a multicolumn GIN index. 

collectMatchBitmap() needs to re-find the index tuple it was previously
looking at, after transiently dropping lock on the index page it's on.
The tuple should still exist and be at its prior position or somewhere
to the right of that, since ginvacuum never removes tuples but
concurrent insertions could add one.  However, there was a thinko in
that logic, to the effect of expecting any inserted tuples to have the
same index "attnum" as what we'd been scanning.  Since there's no
physical separation of tuples with different attnums, it's not terribly
hard to devise scenarios where this fails, leading to transient "lost
saved point in index" errors.  (While I've duplicated this with manual
testing, it seems impossible to make a reproducible test case with our
available testing technology.)

Fix by just continuing the scan when the attnum doesn't match.

While here, improve the error message used if we do fail, so that it
matches the wording used in btree for a similar case.

collectMatchBitmap()'s posting-tree code path was previously not
exercised at all by our regression tests.  While I can't make
a regression test that exhibits the bug, I can at least improve
the code coverage here, so do that.  The test case I made for this
is an extension of one added by 4b754d6c1, so it only works in
HEAD and v13; didn't seem worth trying hard to back-patch it.

Per bug #16595 from Jesse Kinkead.  This has been broken since
multicolumn capability was added to GIN (commit 27cb66fdf),
so back-patch to all supported branches.

Discussion: https://postgr.es/m/16595-633118be8eef9ce2@postgresql.org
This commit is contained in:
Tom Lane 2020-08-27 17:36:13 -04:00
parent fa3bd8d853
commit ff3c16d1e7
1 changed files with 16 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

28
src/backend/access/gin/ginget.c
View File

@ -264,24 +264,28 @@ collectMatchBitmap(GinBtreeData *btree, GinBtreeStack *stack,
/* Search forward to re-find idatum */
for (;;)
{
Datum newDatum;
GinNullCategory newCategory;
if (moveRightIfItNeeded(btree, stack, snapshot) == false)
elog(ERROR, "lost saved point in index"); /* must not happen !!! */
ereport(ERROR,
(errcode(ERRCODE_INTERNAL_ERROR),
errmsg("failed to re-find tuple within index \"%s\"",
RelationGetRelationName(btree->index))));
page = BufferGetPage(stack->buffer);
itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stack->off));
if (gintuple_get_attrnum(btree->ginstate, itup) != attnum)
elog(ERROR, "lost saved point in index"); /* must not happen !!! */
newDatum = gintuple_get_key(btree->ginstate, itup,
&newCategory);
if (gintuple_get_attrnum(btree->ginstate, itup) == attnum)
{
Datum newDatum;
GinNullCategory newCategory;
if (ginCompareEntries(btree->ginstate, attnum,
newDatum, newCategory,
idatum, icategory) == 0)
break; /* Found! */
newDatum = gintuple_get_key(btree->ginstate, itup,
&newCategory);
if (ginCompareEntries(btree->ginstate, attnum,
newDatum, newCategory,
idatum, icategory) == 0)
break; /* Found! */
}
stack->off++;
}