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commit    6be02bbc884b1dd0e0791fc7c7d8280eb2078fce
Author:   Tomas Vondra <tomas.vondra@postgresql.org>
Date:     Wed Mar 26 15:50:13 2025 +0000

    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


 src/backend/access/brin/brin_bloom.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/backend/access/brin/brin_bloom.c b/src/backend/access/brin/brin_bloom.c
index 48027da0225..b93f3952638 100644
--- a/src/backend/access/brin/brin_bloom.c
+++ b/src/backend/access/brin/brin_bloom.c
@@ -663,6 +663,17 @@ 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();
 }
 


[parent: 66235baab72b]