Introduce pg_sequence_read_tuple().

This new function returns the data for the given sequence, i.e.,
the values within the sequence tuple.  Since this function is a
substitute for SELECT from the sequence, the SELECT privilege is
required on the sequence in question.  It returns all NULLs for
sequences for which we lack privileges, other sessions' temporary
sequences, and unlogged sequences on standbys.

This function is primarily intended for use by pg_dump in a
follow-up commit that will use it to optimize dumpSequenceData().
Like pg_sequence_last_value(), which is a support function for the
pg_sequences system view, pg_sequence_read_tuple() is left
undocumented.

Bumps catversion.

Reviewed-by: Michael Paquier, Tom Lane
Discussion: https://postgr.es/m/20240503025140.GA1227404%40nathanxps13

src/backend/commands/sequence.c

@@ -1773,6 +1773,67 @@ pg_sequence_parameters(PG_FUNCTION_ARGS)
 	return HeapTupleGetDatum(heap_form_tuple(tupdesc, values, isnull));
 }
 
+
+/*
+ * Return the sequence tuple.
+ *
+ * This is primarily intended for use by pg_dump to gather sequence data
+ * without needing to individually query each sequence relation.
+ */
+Datum
+pg_sequence_read_tuple(PG_FUNCTION_ARGS)
+{
+	Oid			relid = PG_GETARG_OID(0);
+	SeqTable	elm;
+	Relation	seqrel;
+	Datum		values[SEQ_COL_LASTCOL] = {0};
+	bool		isnull[SEQ_COL_LASTCOL] = {0};
+	TupleDesc	resultTupleDesc;
+	HeapTuple	resultHeapTuple;
+	Datum		result;
+
+	resultTupleDesc = CreateTemplateTupleDesc(SEQ_COL_LASTCOL);
+	TupleDescInitEntry(resultTupleDesc, (AttrNumber) 1, "last_value",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(resultTupleDesc, (AttrNumber) 2, "log_cnt",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(resultTupleDesc, (AttrNumber) 3, "is_called",
+					   BOOLOID, -1, 0);
+	resultTupleDesc = BlessTupleDesc(resultTupleDesc);
+
+	init_sequence(relid, &elm, &seqrel);
+
+	/*
+	 * Return all NULLs for sequences for which we lack privileges, other
+	 * sessions' temporary sequences, and unlogged sequences on standbys.
+	 */
+	if (pg_class_aclcheck(relid, GetUserId(), ACL_SELECT) == ACLCHECK_OK &&
+		!RELATION_IS_OTHER_TEMP(seqrel) &&
+		(RelationIsPermanent(seqrel) || !RecoveryInProgress()))
+	{
+		Buffer		buf;
+		HeapTupleData seqtuple;
+		Form_pg_sequence_data seq;
+
+		seq = read_seq_tuple(seqrel, &buf, &seqtuple);
+
+		values[0] = Int64GetDatum(seq->last_value);
+		values[1] = Int64GetDatum(seq->log_cnt);
+		values[2] = BoolGetDatum(seq->is_called);
+
+		UnlockReleaseBuffer(buf);
+	}
+	else
+		memset(isnull, true, sizeof(isnull));
+
+	sequence_close(seqrel, NoLock);
+
+	resultHeapTuple = heap_form_tuple(resultTupleDesc, values, isnull);
+	result = HeapTupleGetDatum(resultHeapTuple);
+	PG_RETURN_DATUM(result);
+}
+
+
 /*
  * Return the last value from the sequence
  *

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202407252
+#define CATALOG_VERSION_NO	202407311
 
 #endif

src/include/catalog/pg_proc.dat

@@ -3329,6 +3329,12 @@
   proname => 'pg_sequence_last_value', provolatile => 'v', proparallel => 'u',
   prorettype => 'int8', proargtypes => 'regclass',
   prosrc => 'pg_sequence_last_value' },
+{ oid => '9876', descr => 'return sequence tuple, for use by pg_dump',
+  proname => 'pg_sequence_read_tuple', provolatile => 'v', proparallel => 'u',
+  prorettype => 'record', proargtypes => 'regclass',
+  proallargtypes => '{regclass,int8,int8,bool}', proargmodes => '{i,o,o,o}',
+  proargnames => '{sequence_oid,last_value,log_cnt,is_called}',
+  prosrc => 'pg_sequence_read_tuple' },
 
 { oid => '275', descr => 'return the next oid for a system table',
   proname => 'pg_nextoid', provolatile => 'v', proparallel => 'u',

src/test/regress/expected/sequence.out

@@ -839,4 +839,11 @@ SELECT nextval('test_seq1');
        3
 (1 row)
 
+-- pg_sequence_read_tuple
+SELECT * FROM pg_sequence_read_tuple('test_seq1');
+ last_value | log_cnt | is_called 
+------------+---------+-----------
+         10 |      32 | t
+(1 row)
+
 DROP SEQUENCE test_seq1;

src/test/regress/sql/sequence.sql

@@ -413,4 +413,7 @@ SELECT nextval('test_seq1');
 SELECT nextval('test_seq1');
 SELECT nextval('test_seq1');
 
+-- pg_sequence_read_tuple
+SELECT * FROM pg_sequence_read_tuple('test_seq1');
+
 DROP SEQUENCE test_seq1;