Ensure we allocate NAMEDATALEN bytes for names in Index Only Scans

As an optimization, we store "name" columns as cstrings in btree
indexes.

Here we modify it so that Index Only Scans convert these cstrings back
to names with NAMEDATALEN bytes rather than storing the cstring in the
tuple slot, as was happening previously.

Bug: #17855
Reported-by: Alexander Lakhin
Reviewed-by: Alexander Lakhin, Tom Lane
Discussion: https://postgr.es/m/17855-5f523e0f9769a566@postgresql.org
Backpatch-through: 12, all supported versions

Author: david-rowley

src/backend/executor/nodeIndexonlyscan.c

@@ -35,18 +35,20 @@
 #include "access/tableam.h"
 #include "access/tupdesc.h"
 #include "access/visibilitymap.h"
+#include "catalog/pg_type.h"
 #include "executor/executor.h"
 #include "executor/nodeIndexonlyscan.h"
 #include "executor/nodeIndexscan.h"
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "storage/predicate.h"
+#include "utils/builtins.h"
 #include "utils/rel.h"
 
 
 static TupleTableSlot *IndexOnlyNext(IndexOnlyScanState *node);
-static void StoreIndexTuple(TupleTableSlot *slot, IndexTuple itup,
-							TupleDesc itupdesc);
+static void StoreIndexTuple(IndexOnlyScanState *node, TupleTableSlot *slot,
+							IndexTuple itup, TupleDesc itupdesc);
 
 
 /* ----------------------------------------------------------------
@@ -205,7 +207,7 @@ IndexOnlyNext(IndexOnlyScanState *node)
 			ExecForceStoreHeapTuple(scandesc->xs_hitup, slot, false);
 		}
 		else if (scandesc->xs_itup)
-			StoreIndexTuple(slot, scandesc->xs_itup, scandesc->xs_itupdesc);
+			StoreIndexTuple(node, slot, scandesc->xs_itup, scandesc->xs_itupdesc);
 		else
 			elog(ERROR, "no data returned for index-only scan");
 
@@ -263,7 +265,8 @@ IndexOnlyNext(IndexOnlyScanState *node)
  * right now we don't need it elsewhere.
  */
 static void
-StoreIndexTuple(TupleTableSlot *slot, IndexTuple itup, TupleDesc itupdesc)
+StoreIndexTuple(IndexOnlyScanState *node, TupleTableSlot *slot,
+				IndexTuple itup, TupleDesc itupdesc)
 {
 	/*
 	 * Note: we must use the tupdesc supplied by the AM in index_deform_tuple,
@@ -276,6 +279,37 @@ StoreIndexTuple(TupleTableSlot *slot, IndexTuple itup, TupleDesc itupdesc)
 
 	ExecClearTuple(slot);
 	index_deform_tuple(itup, itupdesc, slot->tts_values, slot->tts_isnull);
+
+	/*
+	 * Copy all name columns stored as cstrings back into a NAMEDATALEN byte
+	 * sized allocation.  We mark this branch as unlikely as generally "name"
+	 * is used only for the system catalogs and this would have to be a user
+	 * query running on those or some other user table with an index on a name
+	 * column.
+	 */
+	if (unlikely(node->ioss_NameCStringAttNums != NULL))
+	{
+		int			attcount = node->ioss_NameCStringCount;
+
+		for (int idx = 0; idx < attcount; idx++)
+		{
+			int			attnum = node->ioss_NameCStringAttNums[idx];
+			Name		name;
+
+			/* skip null Datums */
+			if (slot->tts_isnull[attnum])
+				continue;
+
+			/* allocate the NAMEDATALEN and copy the datum into that memory */
+			name = (Name) MemoryContextAlloc(node->ss.ps.ps_ExprContext->ecxt_per_tuple_memory,
+											 NAMEDATALEN);
+
+			/* use namestrcpy to zero-pad all trailing bytes */
+			namestrcpy(name, DatumGetCString(slot->tts_values[attnum]));
+			slot->tts_values[attnum] = NameGetDatum(name);
+		}
+	}
+
 	ExecStoreVirtualTuple(slot);
 }
 
@@ -473,8 +507,11 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 {
 	IndexOnlyScanState *indexstate;
 	Relation	currentRelation;
+	Relation	indexRelation;
 	LOCKMODE	lockmode;
 	TupleDesc	tupDesc;
+	int			indnkeyatts;
+	int			namecount;
 
 	/*
 	 * create state structure
@@ -547,7 +584,8 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 
 	/* Open the index relation. */
 	lockmode = exec_rt_fetch(node->scan.scanrelid, estate)->rellockmode;
-	indexstate->ioss_RelationDesc = index_open(node->indexid, lockmode);
+	indexRelation = index_open(node->indexid, lockmode);
+	indexstate->ioss_RelationDesc = indexRelation;
 
 	/*
 	 * Initialize index-specific scan state
@@ -560,7 +598,7 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 	 * build the index scan keys from the index qualification
 	 */
 	ExecIndexBuildScanKeys((PlanState *) indexstate,
-						   indexstate->ioss_RelationDesc,
+						   indexRelation,
 						   node->indexqual,
 						   false,
 						   &indexstate->ioss_ScanKeys,
@@ -574,7 +612,7 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 	 * any ORDER BY exprs have to be turned into scankeys in the same way
 	 */
 	ExecIndexBuildScanKeys((PlanState *) indexstate,
-						   indexstate->ioss_RelationDesc,
+						   indexRelation,
 						   node->indexorderby,
 						   true,
 						   &indexstate->ioss_OrderByKeys,
@@ -603,6 +641,49 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 		indexstate->ioss_RuntimeContext = NULL;
 	}
 
+	indexstate->ioss_NameCStringAttNums = NULL;
+	indnkeyatts = indexRelation->rd_index->indnkeyatts;
+	namecount = 0;
+
+	/*
+	 * The "name" type for btree uses text_ops which results in storing
+	 * cstrings in the indexed keys rather than names.  Here we detect that in
+	 * a generic way in case other index AMs want to do the same optimization.
+	 * Check for opclasses with an opcintype of NAMEOID and an index tuple
+	 * descriptor with CSTRINGOID.  If any of these are found, create an array
+	 * marking the index attribute number of each of them.  StoreIndexTuple()
+	 * handles copying the name Datums into a NAMEDATALEN-byte allocation.
+	 */
+
+	/* First, count the number of such index keys */
+	for (int attnum = 0; attnum < indnkeyatts; attnum++)
+	{
+		if (indexRelation->rd_att->attrs[attnum].atttypid == CSTRINGOID &&
+			indexRelation->rd_opcintype[attnum] == NAMEOID)
+			namecount++;
+	}
+
+	if (namecount > 0)
+	{
+		int			idx = 0;
+
+		/*
+		 * Now create an array to mark the attribute numbers of the keys that
+		 * need to be converted from cstring to name.
+		 */
+		indexstate->ioss_NameCStringAttNums = (AttrNumber *)
+			palloc(sizeof(AttrNumber) * namecount);
+
+		for (int attnum = 0; attnum < indnkeyatts; attnum++)
+		{
+			if (indexRelation->rd_att->attrs[attnum].atttypid == CSTRINGOID &&
+				indexRelation->rd_opcintype[attnum] == NAMEOID)
+				indexstate->ioss_NameCStringAttNums[idx++] = (AttrNumber) attnum;
+		}
+	}
+
+	indexstate->ioss_NameCStringCount = namecount;
+
 	/*
 	 * all done.
 	 */

src/include/catalog/pg_opclass.dat

@@ -91,8 +91,11 @@
 # Here's an ugly little hack to save space in the system catalog indexes.
 # btree doesn't ordinarily allow a storage type different from input type;
 # but cstring and name are the same thing except for trailing padding,
-# and we can safely omit that within an index entry.  So we declare the
-# btree opclass for name as using cstring storage type.
+# so we choose to omit that within an index entry.  Here we declare the
+# btree opclass for name as using cstring storage type.  This does require
+# that we pad the cstring out with the full NAMEDATALEN bytes when performing
+# index-only scans.  See corresponding hacks in ExecInitIndexOnlyScan() and
+# StoreIndexTuple().
 { opcmethod => 'btree', opcname => 'name_ops', opcfamily => 'btree/text_ops',
   opcintype => 'name', opckeytype => 'cstring' },
 

src/include/nodes/execnodes.h

@@ -1690,6 +1690,8 @@ typedef struct IndexScanState
  *		TableSlot		   slot for holding tuples fetched from the table
  *		VMBuffer		   buffer in use for visibility map testing, if any
  *		PscanLen		   size of parallel index-only scan descriptor
+ *		NameCStringAttNums attnums of name typed columns to pad to NAMEDATALEN
+ *		NameCStringCount   number of elements in the NameCStringAttNums array
  * ----------------
  */
 typedef struct IndexOnlyScanState
@@ -1709,6 +1711,8 @@ typedef struct IndexOnlyScanState
 	TupleTableSlot *ioss_TableSlot;
 	Buffer		ioss_VMBuffer;
 	Size		ioss_PscanLen;
+	AttrNumber *ioss_NameCStringAttNums;
+	int			ioss_NameCStringCount;
 } IndexOnlyScanState;
 
 /* ----------------

src/test/regress/expected/index_including.out

@@ -398,3 +398,28 @@ Indexes:
     "tbl_c1_c2_c3_c4_key" UNIQUE CONSTRAINT, btree (c1, c2) INCLUDE (c3, c4)
 
 DROP TABLE tbl;
+/*
+ * 10. Test coverage for names stored as cstrings in indexes
+ */
+CREATE TABLE nametbl (c1 int, c2 name, c3 float);
+CREATE INDEX nametbl_c1_c2_idx ON nametbl (c2, c1) INCLUDE (c3);
+INSERT INTO nametbl VALUES(1, 'two', 3.0);
+VACUUM nametbl;
+SET enable_seqscan = 0;
+-- Ensure we get an index only scan plan
+EXPLAIN (COSTS OFF) SELECT c2, c1, c3 FROM nametbl WHERE c2 = 'two' AND c1 = 1;
+                     QUERY PLAN                     
+----------------------------------------------------
+ Index Only Scan using nametbl_c1_c2_idx on nametbl
+   Index Cond: ((c2 = 'two'::name) AND (c1 = 1))
+(2 rows)
+
+-- Validate the results look sane
+SELECT c2, c1, c3 FROM nametbl WHERE c2 = 'two' AND c1 = 1;
+ c2  | c1 | c3 
+-----+----+----
+ two |  1 |  3
+(1 row)
+
+RESET enable_seqscan;
+DROP TABLE nametbl;

src/test/regress/sql/index_including.sql

@@ -217,3 +217,22 @@ ALTER TABLE tbl ALTER c1 TYPE bigint;
 ALTER TABLE tbl ALTER c3 TYPE bigint;
 \d tbl
 DROP TABLE tbl;
+
+/*
+ * 10. Test coverage for names stored as cstrings in indexes
+ */
+CREATE TABLE nametbl (c1 int, c2 name, c3 float);
+CREATE INDEX nametbl_c1_c2_idx ON nametbl (c2, c1) INCLUDE (c3);
+INSERT INTO nametbl VALUES(1, 'two', 3.0);
+VACUUM nametbl;
+SET enable_seqscan = 0;
+
+-- Ensure we get an index only scan plan
+EXPLAIN (COSTS OFF) SELECT c2, c1, c3 FROM nametbl WHERE c2 = 'two' AND c1 = 1;
+
+-- Validate the results look sane
+SELECT c2, c1, c3 FROM nametbl WHERE c2 = 'two' AND c1 = 1;
+
+RESET enable_seqscan;
+
+DROP TABLE nametbl;