Preserve pg_attribute.attstattarget across REINDEX CONCURRENTLY

For an index, attstattarget can be updated using ALTER INDEX SET
STATISTICS.  This data was lost on the new index after REINDEX
CONCURRENTLY.

The update of this field is done when the old and new indexes are
swapped to make the fix back-patchable.  Another approach we could look
after in the long-term is to change index_create() to pass the wanted
values of attstattarget when creating the new relation, but, as this
would cause an ABI breakage this can be done only on HEAD.

Reported-by: Ronan Dunklau
Author: Michael Paquier
Reviewed-by: Ronan Dunklau, Tomas Vondra
Discussion: https://postgr.es/m/16628084.uLZWGnKmhe@laptop-ronand
Backpatch-through: 12

Author: michaelpq

src/backend/catalog/index.c

@@ -1728,6 +1728,62 @@ index_concurrently_swap(Oid newIndexId, Oid oldIndexId, const char *oldName)
 	/* Copy data of pg_statistic from the old index to the new one */
 	CopyStatistics(oldIndexId, newIndexId);
 
+	/* Copy pg_attribute.attstattarget for each index attribute */
+	{
+		HeapTuple	attrTuple;
+		Relation	pg_attribute;
+		SysScanDesc scan;
+		ScanKeyData key[1];
+
+		pg_attribute = table_open(AttributeRelationId, RowExclusiveLock);
+		ScanKeyInit(&key[0],
+					Anum_pg_attribute_attrelid,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(newIndexId));
+		scan = systable_beginscan(pg_attribute, AttributeRelidNumIndexId,
+								  true, NULL, 1, key);
+
+		while (HeapTupleIsValid((attrTuple = systable_getnext(scan))))
+		{
+			Form_pg_attribute att = (Form_pg_attribute) GETSTRUCT(attrTuple);
+			Datum		repl_val[Natts_pg_attribute];
+			bool		repl_null[Natts_pg_attribute];
+			bool		repl_repl[Natts_pg_attribute];
+			int			attstattarget;
+			HeapTuple	newTuple;
+
+			/* Ignore dropped columns */
+			if (att->attisdropped)
+				continue;
+
+			/*
+			 * Get attstattarget from the old index and refresh the new value.
+			 */
+			attstattarget = get_attstattarget(oldIndexId, att->attnum);
+
+			/* no need for a refresh if both match */
+			if (attstattarget == att->attstattarget)
+				continue;
+
+			memset(repl_val, 0, sizeof(repl_val));
+			memset(repl_null, false, sizeof(repl_null));
+			memset(repl_repl, false, sizeof(repl_repl));
+
+			repl_repl[Anum_pg_attribute_attstattarget - 1] = true;
+			repl_val[Anum_pg_attribute_attstattarget - 1] = Int32GetDatum(attstattarget);
+
+			newTuple = heap_modify_tuple(attrTuple,
+										 RelationGetDescr(pg_attribute),
+										 repl_val, repl_null, repl_repl);
+			CatalogTupleUpdate(pg_attribute, &newTuple->t_self, newTuple);
+
+			heap_freetuple(newTuple);
+		}
+
+		systable_endscan(scan);
+		table_close(pg_attribute, RowExclusiveLock);
+	}
+
 	/* Close relations */
 	table_close(pg_class, RowExclusiveLock);
 	table_close(pg_index, RowExclusiveLock);

src/backend/utils/cache/lsyscache.c

@@ -871,6 +871,33 @@ get_attnum(Oid relid, const char *attname)
 		return InvalidAttrNumber;
 }
 
+/*
+ * get_attstattarget
+ *
+ *		Given the relation id and the attribute number,
+ *		return the "attstattarget" field from the attribute relation.
+ *
+ *		Errors if not found.
+ */
+int
+get_attstattarget(Oid relid, AttrNumber attnum)
+{
+	HeapTuple	tp;
+	Form_pg_attribute att_tup;
+	int			result;
+
+	tp = SearchSysCache2(ATTNUM,
+						 ObjectIdGetDatum(relid),
+						 Int16GetDatum(attnum));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for attribute %d of relation %u",
+			 attnum, relid);
+	att_tup = (Form_pg_attribute) GETSTRUCT(tp);
+	result = att_tup->attstattarget;
+	ReleaseSysCache(tp);
+	return result;
+}
+
 /*
  * get_attgenerated
  *

src/include/utils/lsyscache.h

@@ -87,6 +87,7 @@ extern Oid	get_opfamily_proc(Oid opfamily, Oid lefttype, Oid righttype,
 							  int16 procnum);
 extern char *get_attname(Oid relid, AttrNumber attnum, bool missing_ok);
 extern AttrNumber get_attnum(Oid relid, const char *attname);
+extern int	get_attstattarget(Oid relid, AttrNumber attnum);
 extern char get_attgenerated(Oid relid, AttrNumber attnum);
 extern Oid	get_atttype(Oid relid, AttrNumber attnum);
 extern void get_atttypetypmodcoll(Oid relid, AttrNumber attnum,

src/test/regress/expected/create_index.out

@@ -2419,6 +2419,7 @@ CREATE UNIQUE INDEX concur_exprs_index_pred ON concur_exprs_tab (c1)
 CREATE UNIQUE INDEX concur_exprs_index_pred_2
   ON concur_exprs_tab ((1 / c1))
   WHERE ('-H') >= (c2::TEXT) COLLATE "C";
+ALTER INDEX concur_exprs_index_expr ALTER COLUMN 1 SET STATISTICS 100;
 ANALYZE concur_exprs_tab;
 SELECT starelid::regclass, count(*) FROM pg_statistic WHERE starelid IN (
   'concur_exprs_index_expr'::regclass,
@@ -2498,6 +2499,20 @@ SELECT starelid::regclass, count(*) FROM pg_statistic WHERE starelid IN (
  concur_exprs_index_expr |     1
 (1 row)
 
+-- attstattarget should remain intact
+SELECT attrelid::regclass, attnum, attstattarget
+  FROM pg_attribute WHERE attrelid IN (
+    'concur_exprs_index_expr'::regclass,
+    'concur_exprs_index_pred'::regclass,
+    'concur_exprs_index_pred_2'::regclass)
+  ORDER BY 'concur_exprs_index_expr'::regclass::text, attnum;
+         attrelid          | attnum | attstattarget 
+---------------------------+--------+---------------
+ concur_exprs_index_expr   |      1 |           100
+ concur_exprs_index_pred   |      1 |            -1
+ concur_exprs_index_pred_2 |      1 |            -1
+(3 rows)
+
 DROP TABLE concur_exprs_tab;
 -- Temporary tables and on-commit actions, where CONCURRENTLY is ignored.
 -- ON COMMIT PRESERVE ROWS, the default.

src/test/regress/sql/create_index.sql

@@ -1003,6 +1003,7 @@ CREATE UNIQUE INDEX concur_exprs_index_pred ON concur_exprs_tab (c1)
 CREATE UNIQUE INDEX concur_exprs_index_pred_2
   ON concur_exprs_tab ((1 / c1))
   WHERE ('-H') >= (c2::TEXT) COLLATE "C";
+ALTER INDEX concur_exprs_index_expr ALTER COLUMN 1 SET STATISTICS 100;
 ANALYZE concur_exprs_tab;
 SELECT starelid::regclass, count(*) FROM pg_statistic WHERE starelid IN (
   'concur_exprs_index_expr'::regclass,
@@ -1027,6 +1028,13 @@ SELECT starelid::regclass, count(*) FROM pg_statistic WHERE starelid IN (
   'concur_exprs_index_pred'::regclass,
   'concur_exprs_index_pred_2'::regclass)
   GROUP BY starelid ORDER BY starelid::regclass::text;
+-- attstattarget should remain intact
+SELECT attrelid::regclass, attnum, attstattarget
+  FROM pg_attribute WHERE attrelid IN (
+    'concur_exprs_index_expr'::regclass,
+    'concur_exprs_index_pred'::regclass,
+    'concur_exprs_index_pred_2'::regclass)
+  ORDER BY 'concur_exprs_index_expr'::regclass::text, attnum;
 DROP TABLE concur_exprs_tab;
 
 -- Temporary tables and on-commit actions, where CONCURRENTLY is ignored.