Fix handling of empty ranges and NULLs in BRIN

BRIN indexes did not properly distinguish between summaries for empty
(no rows) and all-NULL ranges, treating them as essentially the same
thing. Summaries were initialized with allnulls=true, and opclasses
simply reset allnulls to false when processing the first non-NULL value.
This however produces incorrect results if the range starts with a NULL
value (or a sequence of NULL values), in which case we forget the range
contains NULL values when adding the first non-NULL value.

This happens because the allnulls flag is used for two separate
purposes - to mark empty ranges (not representing any rows yet) and
ranges containing only NULL values.

Opclasses don't know which of these cases it is, and so don't know
whether to set hasnulls=true. Setting the flag in both cases would make
it correct, but it would also make BRIN indexes useless for queries with
IS NULL clauses. All ranges start empty (and thus allnulls=true), so all
ranges would end up with either allnulls=true or hasnulls=true.

The severity of the issue is somewhat reduced by the fact that it only
happens when adding values to an existing summary with allnulls=true.
This can happen e.g. for small tables (because a summary for the first
range exists for all BRIN indexes), or for tables with large fraction of
NULL values in the indexed columns.

Bulk summarization (e.g. during CREATE INDEX or automatic summarization)
that processes all values at once is not affected by this issue. In this
case the flags were updated in a slightly different way, not forgetting
the NULL values.

To identify empty ranges we use a new flag, stored in an unused bit in
the BRIN tuple header so the on-disk format remains the same. A matching
flag is added to BrinMemTuple, into a 3B gap after bt_placeholder.
That means there's no risk of ABI breakage, although we don't actually
pass the BrinMemTuple to any public API.

We could also skip storing index tuples for empty summaries, but then
we'd have to always process such ranges - even if there are no rows in
large parts of the table (e.g. after a bulk DELETE), it would still
require reading the pages etc. So we store them, but ignore them when
building the bitmap.

Backpatch to 11. The issue exists since BRIN indexes were introduced in
9.5, but older releases are already EOL.

Backpatch-through: 11
Reviewed-by: Justin Pryzby, Matthias van de Meent, Alvaro Herrera
Discussion: https://postgr.es/m/402430e4-7d9d-6cf1-09ef-464d80afff3b@enterprisedb.com

Author: tvondra

src/backend/access/brin/brin.c

@@ -592,6 +592,17 @@ bringetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 
 					bval = &dtup->bt_columns[attno - 1];
 
+					/*
+					 * If the BRIN tuple indicates that this range is empty,
+					 * we can skip it: there's nothing to match.  We don't
+					 * need to examine the next columns.
+					 */
+					if (dtup->bt_empty_range)
+					{
+						addrange = false;
+						break;
+					}
+
 					/*
 					 * First check if there are any IS [NOT] NULL scan keys,
 					 * and if we're violating them. In that case we can
@@ -1606,6 +1617,64 @@ union_tuples(BrinDesc *bdesc, BrinMemTuple *a, BrinTuple *b)
 	db = brin_deform_tuple(bdesc, b, NULL);
 	MemoryContextSwitchTo(oldcxt);
 
+	/*
+	 * Check if the ranges are empty.
+	 *
+	 * If at least one of them is empty, we don't need to call per-key union
+	 * functions at all. If "b" is empty, we just use "a" as the result (it
+	 * might be empty fine, but that's fine). If "a" is empty but "b" is not,
+	 * we use "b" as the result (but we have to copy the data into "a" first).
+	 *
+	 * Only when both ranges are non-empty, we actually do the per-key merge.
+	 */
+
+	/* If "b" is empty - ignore it and just use "a" (even if it's empty etc.). */
+	if (db->bt_empty_range)
+	{
+		/* skip the per-key merge */
+		MemoryContextDelete(cxt);
+		return;
+	}
+
+	/*
+	 * Now we know "b" is not empty. If "a" is empty, then "b" is the result.
+	 * But we need to copy the data from "b" to "a" first, because that's how
+	 * we pass result out.
+	 *
+	 * We have to copy all the global/per-key flags etc. too.
+	 */
+	if (a->bt_empty_range)
+	{
+		for (keyno = 0; keyno < bdesc->bd_tupdesc->natts; keyno++)
+		{
+			int			i;
+			BrinValues *col_a = &a->bt_columns[keyno];
+			BrinValues *col_b = &db->bt_columns[keyno];
+			BrinOpcInfo *opcinfo = bdesc->bd_info[keyno];
+
+			col_a->bv_allnulls = col_b->bv_allnulls;
+			col_a->bv_hasnulls = col_b->bv_hasnulls;
+
+			/* If "b" has no data, we're done. */
+			if (col_b->bv_allnulls)
+				continue;
+
+			for (i = 0; i < opcinfo->oi_nstored; i++)
+				col_a->bv_values[i] =
+					datumCopy(col_b->bv_values[i],
+							  opcinfo->oi_typcache[i]->typbyval,
+							  opcinfo->oi_typcache[i]->typlen);
+		}
+
+		/* "a" started empty, but "b" was not empty, so remember that */
+		a->bt_empty_range = false;
+
+		/* skip the per-key merge */
+		MemoryContextDelete(cxt);
+		return;
+	}
+
+	/* Now we know neither range is empty. */
 	for (keyno = 0; keyno < bdesc->bd_tupdesc->natts; keyno++)
 	{
 		FmgrInfo   *unionFn;
@@ -1711,7 +1780,9 @@ add_values_to_range(Relation idxRel, BrinDesc *bdesc, BrinMemTuple *dtup,
 					Datum *values, bool *nulls)
 {
 	int			keyno;
-	bool		modified = false;
+
+	/* If the range starts empty, we're certainly going to modify it. */
+	bool		modified = dtup->bt_empty_range;
 
 	/*
 	 * Compare the key values of the new tuple to the stored index values; our
@@ -1725,9 +1796,24 @@ add_values_to_range(Relation idxRel, BrinDesc *bdesc, BrinMemTuple *dtup,
 		Datum		result;
 		BrinValues *bval;
 		FmgrInfo   *addValue;
+		bool		has_nulls;
 
 		bval = &dtup->bt_columns[keyno];
 
+		/*
+		 * Does the range have actual NULL values? Either of the flags can
+		 * be set, but we ignore the state before adding first row.
+		 *
+		 * We have to remember this, because we'll modify the flags and we
+		 * need to know if the range started as empty.
+		 */
+		has_nulls = ((!dtup->bt_empty_range) &&
+					 (bval->bv_hasnulls || bval->bv_allnulls));
+
+		/*
+		 * If the value we're adding is NULL, handle it locally. Otherwise
+		 * call the BRIN_PROCNUM_ADDVALUE procedure.
+		 */
 		if (bdesc->bd_info[keyno]->oi_regular_nulls && nulls[keyno])
 		{
 			/*
@@ -1753,8 +1839,33 @@ add_values_to_range(Relation idxRel, BrinDesc *bdesc, BrinMemTuple *dtup,
 								   nulls[keyno]);
 		/* if that returned true, we need to insert the updated tuple */
 		modified |= DatumGetBool(result);
+
+		/*
+		 * If the range was had actual NULL values (i.e. did not start empty),
+		 * make sure we don't forget about the NULL values. Either the allnulls
+		 * flag is still set to true, or (if the opclass cleared it) we need to
+		 * set hasnulls=true.
+		 *
+		 * XXX This can only happen when the opclass modified the tuple, so the
+		 * modified flag should be set.
+		 */
+		if (has_nulls && !(bval->bv_hasnulls || bval->bv_allnulls))
+		{
+			Assert(modified);
+			bval->bv_hasnulls = true;
+		}
 	}
 
+	/*
+	 * After updating summaries for all the keys, mark it as not empty.
+	 *
+	 * If we're actually changing the flag value (i.e. tuple started as empty),
+	 * we should have modified the tuple. So we should not see empty range that
+	 * was not modified.
+	 */
+	Assert(!dtup->bt_empty_range || modified);
+	dtup->bt_empty_range = false;
+
 	return modified;
 }
 

src/backend/access/brin/brin_tuple.c

@@ -372,6 +372,9 @@ brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno, BrinMemTuple *tuple,
 	if (tuple->bt_placeholder)
 		rettuple->bt_info |= BRIN_PLACEHOLDER_MASK;
 
+	if (tuple->bt_empty_range)
+		rettuple->bt_info |= BRIN_EMPTY_RANGE_MASK;
+
 	*size = len;
 	return rettuple;
 }
@@ -399,7 +402,7 @@ brin_form_placeholder_tuple(BrinDesc *brdesc, BlockNumber blkno, Size *size)
 	rettuple = palloc0(len);
 	rettuple->bt_blkno = blkno;
 	rettuple->bt_info = hoff;
-	rettuple->bt_info |= BRIN_NULLS_MASK | BRIN_PLACEHOLDER_MASK;
+	rettuple->bt_info |= BRIN_NULLS_MASK | BRIN_PLACEHOLDER_MASK | BRIN_EMPTY_RANGE_MASK;
 
 	bitP = ((bits8 *) ((char *) rettuple + SizeOfBrinTuple)) - 1;
 	bitmask = HIGHBIT;
@@ -489,6 +492,8 @@ brin_new_memtuple(BrinDesc *brdesc)
 	dtup->bt_allnulls = palloc(sizeof(bool) * brdesc->bd_tupdesc->natts);
 	dtup->bt_hasnulls = palloc(sizeof(bool) * brdesc->bd_tupdesc->natts);
 
+	dtup->bt_empty_range = true;
+
 	dtup->bt_context = AllocSetContextCreate(CurrentMemoryContext,
 											 "brin dtuple",
 											 ALLOCSET_DEFAULT_SIZES);
@@ -527,6 +532,8 @@ brin_memtuple_initialize(BrinMemTuple *dtuple, BrinDesc *brdesc)
 		currdatum += sizeof(Datum) * brdesc->bd_info[i]->oi_nstored;
 	}
 
+	dtuple->bt_empty_range = true;
+
 	return dtuple;
 }
 
@@ -560,6 +567,11 @@ brin_deform_tuple(BrinDesc *brdesc, BrinTuple *tuple, BrinMemTuple *dMemtuple)
 
 	if (BrinTupleIsPlaceholder(tuple))
 		dtup->bt_placeholder = true;
+
+	/* ranges start as empty, depends on the BrinTuple */
+	if (!BrinTupleIsEmptyRange(tuple))
+		dtup->bt_empty_range = false;
+
 	dtup->bt_blkno = tuple->bt_blkno;
 
 	values = dtup->bt_values;

src/include/access/brin_tuple.h

@@ -44,6 +44,7 @@ typedef struct BrinValues
 typedef struct BrinMemTuple
 {
 	bool		bt_placeholder; /* this is a placeholder tuple */
+	bool		bt_empty_range;	/* range represents no tuples */
 	BlockNumber bt_blkno;		/* heap blkno that the tuple is for */
 	MemoryContext bt_context;	/* memcxt holding the bt_columns values */
 	/* output arrays for brin_deform_tuple: */
@@ -69,7 +70,7 @@ typedef struct BrinTuple
 	 *
 	 * 7th (high) bit: has nulls
 	 * 6th bit: is placeholder tuple
-	 * 5th bit: unused
+	 * 5th bit: range is empty
 	 * 4-0 bit: offset of data
 	 * ---------------
 	 */
@@ -82,13 +83,14 @@ typedef struct BrinTuple
  * bt_info manipulation macros
  */
 #define BRIN_OFFSET_MASK		0x1F
-/* bit 0x20 is not used at present */
+#define BRIN_EMPTY_RANGE_MASK	0x20
 #define BRIN_PLACEHOLDER_MASK	0x40
 #define BRIN_NULLS_MASK			0x80
 
 #define BrinTupleDataOffset(tup)	((Size) (((BrinTuple *) (tup))->bt_info & BRIN_OFFSET_MASK))
 #define BrinTupleHasNulls(tup)	(((((BrinTuple *) (tup))->bt_info & BRIN_NULLS_MASK)) != 0)
 #define BrinTupleIsPlaceholder(tup) (((((BrinTuple *) (tup))->bt_info & BRIN_PLACEHOLDER_MASK)) != 0)
+#define BrinTupleIsEmptyRange(tup) (((((BrinTuple *) (tup))->bt_info & BRIN_EMPTY_RANGE_MASK)) != 0)
 
 
 extern BrinTuple *brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno,

src/test/modules/brin/expected/summarization-and-inprogress-insertion.out

@@ -4,7 +4,7 @@ starting permutation: s2check s1b s2b s1i s2summ s1c s2c s2check
 step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
 itemoffset|blknum|attnum|allnulls|hasnulls|placeholder|value   
 ----------+------+------+--------+--------+-----------+--------
-         1|     0|     1|f       |f       |f          |{1 .. 1}
+         1|     0|     1|f       |t       |f          |{1 .. 1}
 (1 row)
 
 step s1b: BEGIN ISOLATION LEVEL REPEATABLE READ;
@@ -26,7 +26,7 @@ step s2c: COMMIT;
 step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
 itemoffset|blknum|attnum|allnulls|hasnulls|placeholder|value      
 ----------+------+------+--------+--------+-----------+-----------
-         1|     0|     1|f       |f       |f          |{1 .. 1}   
+         1|     0|     1|f       |t       |f          |{1 .. 1}   
          2|     1|     1|f       |f       |f          |{1 .. 1000}
 (2 rows)
 
@@ -35,7 +35,7 @@ starting permutation: s2check s1b s1i s2vacuum s1c s2check
 step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
 itemoffset|blknum|attnum|allnulls|hasnulls|placeholder|value   
 ----------+------+------+--------+--------+-----------+--------
-         1|     0|     1|f       |f       |f          |{1 .. 1}
+         1|     0|     1|f       |t       |f          |{1 .. 1}
 (1 row)
 
 step s1b: BEGIN ISOLATION LEVEL REPEATABLE READ;
@@ -45,7 +45,7 @@ step s1c: COMMIT;
 step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
 itemoffset|blknum|attnum|allnulls|hasnulls|placeholder|value      
 ----------+------+------+--------+--------+-----------+-----------
-         1|     0|     1|f       |f       |f          |{1 .. 1}   
+         1|     0|     1|f       |t       |f          |{1 .. 1}   
          2|     1|     1|f       |f       |f          |{1 .. 1000}
 (2 rows)
 

src/test/modules/brin/specs/summarization-and-inprogress-insertion.spec

@@ -9,6 +9,7 @@ setup
     ) WITH (fillfactor=10);
     CREATE INDEX brinidx ON brin_iso USING brin (value) WITH (pages_per_range=1);
     -- this fills the first page
+    INSERT INTO brin_iso VALUES (NULL);
     DO $$
     DECLARE curtid tid;
     BEGIN