Introduce sequence_*() access functions

Similarly to tables and indexes, these functions are able to open
relations with a sequence relkind, which is useful to make a distinction
with the other relation kinds.  Previously, commands/sequence.c used a
mix of table_{close,open}() and relation_{close,open}() routines when
manipulating sequence relations, so this clarifies the code.

A direct effect of this change is to align the error messages produced
when attempting DDLs for sequences on relations with an unexpected
relkind, like a table or an index with ALTER SEQUENCE, providing an
extra error detail about the relkind of the relation used in the DDL
query.

Author: Michael Paquier
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/ZWlohtKAs0uVVpZ3@paquier.xyz

src/backend/access/Makefile

@@ -9,6 +9,6 @@ top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
 SUBDIRS	    = brin common gin gist hash heap index nbtree rmgrdesc spgist \
-			  table tablesample transam
+			  sequence table tablesample transam
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/access/meson.build

@@ -9,6 +9,7 @@ subdir('heap')
 subdir('index')
 subdir('nbtree')
 subdir('rmgrdesc')
+subdir('sequence')
 subdir('spgist')
 subdir('table')
 subdir('tablesample')

src/backend/access/sequence/Makefile

@@ -0,0 +1,17 @@
+#-------------------------------------------------------------------------
+#
+# Makefile
+#    Makefile for access/sequence
+#
+# IDENTIFICATION
+#    src/backend/access/sequence/Makefile
+#
+#-------------------------------------------------------------------------
+
+subdir = src/backend/access/sequence
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+
+OBJS = sequence.o
+
+include $(top_srcdir)/src/backend/common.mk

src/backend/access/sequence/meson.build

@@ -0,0 +1,5 @@
+# Copyright (c) 2022-2024, PostgreSQL Global Development Group
+
+backend_sources += files(
+  'sequence.c',
+)

src/backend/access/sequence/sequence.c

@@ -0,0 +1,78 @@
+/*-------------------------------------------------------------------------
+ *
+ * sequence.c
+ *	  Generic routines for sequence-related code.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/sequence/sequence.c
+ *
+ *
+ * NOTES
+ *	  This file contains sequence_ routines that implement access to sequences
+ *	  (in contrast to other relation types like indexes).
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/relation.h"
+#include "access/sequence.h"
+#include "storage/lmgr.h"
+
+static inline void validate_relation_kind(Relation r);
+
+/* ----------------
+ *		sequence_open - open a sequence relation by relation OID
+ *
+ *		This is essentially relation_open plus check that the relation
+ *		is a sequence.
+ * ----------------
+ */
+Relation
+sequence_open(Oid relationId, LOCKMODE lockmode)
+{
+	Relation	r;
+
+	r = relation_open(relationId, lockmode);
+
+	validate_relation_kind(r);
+
+	return r;
+}
+
+/* ----------------
+ *		sequence_close - close a sequence
+ *
+ *		If lockmode is not "NoLock", we then release the specified lock.
+ *
+ *		Note that it is often sensible to hold a lock beyond relation_close;
+ *		in that case, the lock is released automatically at xact end.
+ * ----------------
+ */
+void
+sequence_close(Relation relation, LOCKMODE lockmode)
+{
+	relation_close(relation, lockmode);
+}
+
+/* ----------------
+ *		validate_relation_kind - check the relation's kind
+ *
+ *		Make sure relkind is from an index
+ * ----------------
+ */
+static inline void
+validate_relation_kind(Relation r)
+{
+	if (r->rd_rel->relkind != RELKIND_SEQUENCE)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("cannot open relation \"%s\"",
+						RelationGetRelationName(r)),
+				 errdetail_relkind_not_supported(r->rd_rel->relkind)));
+}

src/backend/commands/sequence.c

@@ -18,6 +18,7 @@
 #include "access/htup_details.h"
 #include "access/multixact.h"
 #include "access/relation.h"
+#include "access/sequence.h"
 #include "access/table.h"
 #include "access/transam.h"
 #include "access/xact.h"
@@ -208,7 +209,7 @@ DefineSequence(ParseState *pstate, CreateSeqStmt *seq)
 	seqoid = address.objectId;
 	Assert(seqoid != InvalidOid);
 
-	rel = table_open(seqoid, AccessExclusiveLock);
+	rel = sequence_open(seqoid, AccessExclusiveLock);
 	tupDesc = RelationGetDescr(rel);
 
 	/* now initialize the sequence's data */
@@ -219,7 +220,7 @@ DefineSequence(ParseState *pstate, CreateSeqStmt *seq)
 	if (owned_by)
 		process_owned_by(rel, owned_by, seq->for_identity);
 
-	table_close(rel, NoLock);
+	sequence_close(rel, NoLock);
 
 	/* fill in pg_sequence */
 	rel = table_open(SequenceRelationId, RowExclusiveLock);
@@ -324,7 +325,7 @@ ResetSequence(Oid seq_relid)
 	/* Note that we do not change the currval() state */
 	elm->cached = elm->last;
 
-	relation_close(seq_rel, NoLock);
+	sequence_close(seq_rel, NoLock);
 }
 
 /*
@@ -531,7 +532,7 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 	ObjectAddressSet(address, RelationRelationId, relid);
 
 	table_close(rel, RowExclusiveLock);
-	relation_close(seqrel, NoLock);
+	sequence_close(seqrel, NoLock);
 
 	return address;
 }
@@ -555,7 +556,7 @@ SequenceChangePersistence(Oid relid, char newrelpersistence)
 	fill_seq_with_data(seqrel, &seqdatatuple);
 	UnlockReleaseBuffer(buf);
 
-	relation_close(seqrel, NoLock);
+	sequence_close(seqrel, NoLock);
 }
 
 void
@@ -662,7 +663,7 @@ nextval_internal(Oid relid, bool check_permissions)
 		Assert(elm->last_valid);
 		Assert(elm->increment != 0);
 		elm->last += elm->increment;
-		relation_close(seqrel, NoLock);
+		sequence_close(seqrel, NoLock);
 		last_used_seq = elm;
 		return elm->last;
 	}
@@ -849,7 +850,7 @@ nextval_internal(Oid relid, bool check_permissions)
 
 	UnlockReleaseBuffer(buf);
 
-	relation_close(seqrel, NoLock);
+	sequence_close(seqrel, NoLock);
 
 	return result;
 }
@@ -880,7 +881,7 @@ currval_oid(PG_FUNCTION_ARGS)
 
 	result = elm->last;
 
-	relation_close(seqrel, NoLock);
+	sequence_close(seqrel, NoLock);
 
 	PG_RETURN_INT64(result);
 }
@@ -915,7 +916,7 @@ lastval(PG_FUNCTION_ARGS)
 						RelationGetRelationName(seqrel))));
 
 	result = last_used_seq->last;
-	relation_close(seqrel, NoLock);
+	sequence_close(seqrel, NoLock);
 
 	PG_RETURN_INT64(result);
 }
@@ -1030,7 +1031,7 @@ do_setval(Oid relid, int64 next, bool iscalled)
 
 	UnlockReleaseBuffer(buf);
 
-	relation_close(seqrel, NoLock);
+	sequence_close(seqrel, NoLock);
 }
 
 /*
@@ -1095,7 +1096,7 @@ lock_and_open_sequence(SeqTable seq)
 	}
 
 	/* We now know we have the lock, and can safely open the rel */
-	return relation_open(seq->relid, NoLock);
+	return sequence_open(seq->relid, NoLock);
 }
 
 /*
@@ -1152,12 +1153,6 @@ init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel)
 	 */
 	seqrel = lock_and_open_sequence(elm);
 
-	if (seqrel->rd_rel->relkind != RELKIND_SEQUENCE)
-		ereport(ERROR,
-				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-				 errmsg("\"%s\" is not a sequence",
-						RelationGetRelationName(seqrel))));
-
 	/*
 	 * If the sequence has been transactionally replaced since we last saw it,
 	 * discard any cached-but-unissued values.  We do not touch the currval()
@@ -1803,7 +1798,7 @@ pg_sequence_last_value(PG_FUNCTION_ARGS)
 	result = seq->last_value;
 
 	UnlockReleaseBuffer(buf);
-	relation_close(seqrel, NoLock);
+	sequence_close(seqrel, NoLock);
 
 	if (is_called)
 		PG_RETURN_INT64(result);

src/include/access/sequence.h

@@ -0,0 +1,23 @@
+/*-------------------------------------------------------------------------
+ *
+ * sequence.h
+ *	  Generic routines for sequence-related code.
+ *
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/access/sequence.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef ACCESS_SEQUENCE_H
+#define ACCESS_SEQUENCE_H
+
+#include "storage/lockdefs.h"
+#include "utils/relcache.h"
+
+extern Relation sequence_open(Oid relationId, LOCKMODE lockmode);
+extern void sequence_close(Relation relation, LOCKMODE lockmode);
+
+#endif							/* ACCESS_SEQUENCE_H */

src/test/regress/expected/sequence.out

@@ -313,7 +313,8 @@ ALTER SEQUENCE IF EXISTS sequence_test2 RESTART WITH 24
   INCREMENT BY 4 MAXVALUE 36 MINVALUE 5 CYCLE;
 NOTICE:  relation "sequence_test2" does not exist, skipping
 ALTER SEQUENCE serialTest1 CYCLE;  -- error, not a sequence
-ERROR:  "serialtest1" is not a sequence
+ERROR:  cannot open relation "serialtest1"
+DETAIL:  This operation is not supported for tables.
 CREATE SEQUENCE sequence_test2 START WITH 32;
 CREATE SEQUENCE sequence_test4 INCREMENT BY -1;
 SELECT nextval('sequence_test2');