Initialize bs_reltuples in parallel GIN builds

Index builds update pg_class.reltuples for the table. In parallel GIN
builds, workers track the number of processed rows, and report it to
the leader, who then updates the pg_class with a total. However,
gin_parallel_build_main failed to initialize the bs_reltuples field,
leaving it set to whatever happens to be on the stack (which may be
bogus values like Infinity or NaN, or just impossibly high values).

If such values get reported to the leader and stored in pg_class, that
can have serious consequences. The pg_class.reltuples field is used to
decide when a table is due for autovacuum or autoanalyze, and if it
happens to be set to a bogus value, that may never happen. The field is
also used by the optimizer when calculating costs.

Fixed by initializing bs_reltuples together with the rest of the build
state. The bs_numtuples was initialized later, but it seems cleaner to
just initialize all the fields at once.

After a bogus value gets persisted in pg_class, affected systems are
unlikely to self-heal. That would require an ANALYZE, but preventing
that is one of the consequences. We have considered forcing autoanalyze
in these cases, but there's not a good way to reliably identify bogus
values (except for a small minority like Infitiny/NaN).

A manual ANALYZE on (possibly) affected tables is the only solution.

Backpatch to 18, where parallel GIN builds were introduced.

Reported-by: Jan Nidzwetzki <jan@planetscale.com>
Discussion: https://postgr.es/m/518BA772-8026-412A-AA8F-A7FE4C6B3717@planetscale.com
Backpatch-through: 18

diff --git a/src/backend/access/gin/gininsert.c b/src/backend/access/gin/gininsert.c
index cb9ed3b563c6feefdf284d79d41c91752ca9b33b..16abb76c001b11d269f4700ee416713752194c27 100644 (file)
--- a/src/backend/access/gin/gininsert.c
+++ b/src/backend/access/gin/gininsert.c
@@ -1875,9 +1875,6 @@ _gin_process_worker_data(GinBuildState *state, Tuplesortstate *worker_sort,
 
        tuplesort_performsort(state->bs_worker_sort);
 
-       /* reset the number of GIN tuples produced by this worker */
-       state->bs_numtuples = 0;
-
        if (progress)
                pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
                                                                         PROGRESS_GIN_PHASE_MERGE_1);
@@ -2158,6 +2155,11 @@ _gin_parallel_build_main(dsm_segment *seg, shm_toc *toc)
        /* initialize the GIN build state */
        initGinState(&buildstate.ginstate, indexRel);
        buildstate.indtuples = 0;
+
+       /* Initialize counters used to report tuple counts to the leader */
+       buildstate.bs_numtuples = 0;
+       buildstate.bs_reltuples = 0;
+
        memset(&buildstate.buildStats, 0, sizeof(GinStatsData));
        memset(&buildstate.tid, 0, sizeof(ItemPointerData));