BRIN revmap pages are not standard pages ...

alvherre · alvherre · commit 7403561c0f6a · 2017-01-09T18:19:29.000-03:00
... and therefore we ought not to tell XLogRegisterBuffer the opposite, when writing XLog for a brin update that moves the index tuple to a different page. Otherwise, xlog insertion would try to "compress the hole" when producing a full-page image for it; but since we don't update pd_lower/upper, the hole covers the whole page. On WAL replay, the revmap page becomes empty and so the entire portion of the index is useless and needs to be recomputed. This is low-probability: a BRIN update only moves an index tuple to a different page when the summary tuple is larger than the existing one, which doesn't happen with fixed-width datatypes. Also, the revmap page must be first after a checkpoint. Report and patch: Kuntal Ghosh Bug is alleged to have detected by a WAL-consistency-checking tool. Discussion: https://postgr.es/m/CAGz5QCJ=00UQjScSEFbV=0qO5ShTZB9WWz_Fm7+Wd83zPs9Geg@mail.gmail.com I posted a test case demonstrating the problem, but I'm refraining from adding it to the test suite; if the WAL consistency tool makes it in, that will be a better way to catch this from regressing. (We should definitely have someting that causes not-same-page updates, though.)
diff --git a/src/backend/access/brin/brin_pageops.c b/src/backend/access/brin/brin_pageops.c
@@ -287,7 +287,7 @@ brin_doupdate(Relation idxrel, BlockNumber pagesPerRange,
 			XLogRegisterBufData(0, (char *) newtup, newsz);
 
 			/* revmap page */
-			XLogRegisterBuffer(1, revmapbuf, REGBUF_STANDARD);
+			XLogRegisterBuffer(1, revmapbuf, 0);
 
 			/* old page */
 			XLogRegisterBuffer(2, oldbuf, REGBUF_STANDARD);
