Keep the decompressed filter in brin_bloom_union

The brin_bloom_union() function combines two BRIN summaries, by merging
one filter into the other. With bloom, we have to decompress the filters
first, but the function failed to update the summary to store the merged
filter. As a consequence, the index may be missing some of the data, and
return false negatives.

This issue exists since BRIN bloom indexes were introduced in Postgres
14, but at that point the union function was called only when two
sessions happened to summarize a range concurrently, which is rare. It
got much easier to hit in 17, as parallel builds use the union function
to merge summaries built by workers.

Fixed by storing a pointer to the decompressed filter, and freeing the
original one. Free the second filter too, if it was decompressed. The
freeing is not strictly necessary, because the union is called in
short-lived contexts, but it's tidy.

Backpatch to 14, where BRIN bloom indexes were introduced.

Reported by Arseniy Mukhin, investigation and fix by me.

Reported-by: Arseniy Mukhin
Discussion: https://postgr.es/m/18855-1cf1c8bcc22150e6%40postgresql.org
Backpatch-through: 14

diff --git a/src/backend/access/brin/brin_bloom.c b/src/backend/access/brin/brin_bloom.c
index 0bcf8e582c8eb0cbe1ca15fd8716b4087ec936d5..4526038b26d11d33ba6d6d8706fcaaa50b3aeb94 100644 (file)
--- a/src/backend/access/brin/brin_bloom.c
+++ b/src/backend/access/brin/brin_bloom.c
@@ -662,6 +662,17 @@ brin_bloom_union(PG_FUNCTION_ARGS)
        /* update the number of bits set in the filter */
        filter_a->nbits_set = pg_popcount((const char *) filter_a->data, nbytes);
 
+       /* if we decompressed filter_a, update the summary */
+       if (PointerGetDatum(filter_a) != col_a->bv_values[0])
+       {
+               pfree(DatumGetPointer(col_a->bv_values[0]));
+               col_a->bv_values[0] = PointerGetDatum(filter_a);
+       }
+
+       /* also free filter_b, if it was decompressed */
+       if (PointerGetDatum(filter_b) != col_b->bv_values[0])
+               pfree(filter_b);
+
        PG_RETURN_VOID();
 }