Avoid unnecessary catalog updates in ALTER SEQUENCE

ALTER SEQUENCE can do nontransactional changes to the sequence (RESTART
clause) and transactional updates to the pg_sequence catalog (most other
clauses).  When just calling RESTART, the code would still needlessly do
a catalog update without any changes.  This would entangle that
operation in the concurrency issues of a catalog update (causing either
locking or concurrency errors, depending on how that issue is to be
resolved).

Fix by keeping track during options parsing whether a catalog update is
needed, and skip it if not.

Reported-by: Jason Petersen <jason@citusdata.com>

src/backend/commands/sequence.c

@@ -101,6 +101,7 @@ static Form_pg_sequence_data read_seq_tuple(Relation rel,
 static void init_params(ParseState *pstate, List *options, bool for_identity,
 						bool isInit,
 						Form_pg_sequence seqform,
+						bool *changed_seqform,
 						Form_pg_sequence_data seqdataform, List **owned_by);
 static void do_setval(Oid relid, int64 next, bool iscalled);
 static void process_owned_by(Relation seqrel, List *owned_by, bool for_identity);
@@ -115,6 +116,7 @@ DefineSequence(ParseState *pstate, CreateSeqStmt *seq)
 {
 	FormData_pg_sequence seqform;
 	FormData_pg_sequence_data seqdataform;
+	bool		changed_seqform = false; /* not used here */
 	List	   *owned_by;
 	CreateStmt *stmt = makeNode(CreateStmt);
 	Oid			seqoid;
@@ -153,7 +155,7 @@ DefineSequence(ParseState *pstate, CreateSeqStmt *seq)
 	}
 
 	/* Check and set all option values */
-	init_params(pstate, seq->options, seq->for_identity, true, &seqform, &seqdataform, &owned_by);
+	init_params(pstate, seq->options, seq->for_identity, true, &seqform, &changed_seqform, &seqdataform, &owned_by);
 
 	/*
 	 * Create relation (and fill value[] and null[] for the tuple)
@@ -418,6 +420,7 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 	Form_pg_sequence seqform;
 	Form_pg_sequence_data seqdata;
 	FormData_pg_sequence_data newseqdata;
+	bool		changed_seqform = false;
 	List	   *owned_by;
 	ObjectAddress address;
 	Relation	rel;
@@ -443,7 +446,7 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 	/* lock page' buffer and read tuple into new sequence structure */
 	seqdata = read_seq_tuple(seqrel, &buf, &seqdatatuple);
 
-	/* Copy old values of options into workspace */
+	/* Copy old sequence data into workspace */
 	memcpy(&newseqdata, seqdata, sizeof(FormData_pg_sequence_data));
 
 	rel = heap_open(SequenceRelationId, RowExclusiveLock);
@@ -456,7 +459,7 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 	seqform = (Form_pg_sequence) GETSTRUCT(tuple);
 
 	/* Check and set new values */
-	init_params(pstate, stmt->options, stmt->for_identity, false, seqform, &newseqdata, &owned_by);
+	init_params(pstate, stmt->options, stmt->for_identity, false, seqform, &changed_seqform, &newseqdata, &owned_by);
 
 	/* Clear local cache so that we don't think we have cached numbers */
 	/* Note that we do not change the currval() state */
@@ -507,6 +510,7 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 
 	relation_close(seqrel, NoLock);
 
+	if (changed_seqform)
 		CatalogTupleUpdate(rel, &tuple->t_self, tuple);
 	heap_close(rel, RowExclusiveLock);
 
@@ -1213,9 +1217,12 @@ read_seq_tuple(Relation rel, Buffer *buf, HeapTuple seqdatatuple)
 }
 
 /*
- * init_params: process the options list of CREATE or ALTER SEQUENCE,
+ * init_params: process the options list of CREATE or ALTER SEQUENCE, and
- * and store the values into appropriate fields of *new.  Also set
+ * store the values into appropriate fields of seqform, for changes that go
- * *owned_by to any OWNED BY option, or to NIL if there is none.
+ * into the pg_sequence catalog, and seqdataform for changes to the sequence
+ * relation itself.  Set *changed_seqform to true if seqform was changed
+ * (interesting for ALTER SEQUENCE).  Also set *owned_by to any OWNED BY
+ * option, or to NIL if there is none.
  *
  * If isInit is true, fill any unspecified options with default values;
  * otherwise, do not change existing options that aren't explicitly overridden.
@@ -1224,7 +1231,9 @@ static void
 init_params(ParseState *pstate, List *options, bool for_identity,
 			bool isInit,
 			Form_pg_sequence seqform,
-			Form_pg_sequence_data seqdataform, List **owned_by)
+			bool *changed_seqform,
+			Form_pg_sequence_data seqdataform,
+			List **owned_by)
 {
 	DefElem    *as_type = NULL;
 	DefElem    *start_value = NULL;
@@ -1342,6 +1351,8 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 				 defel->defname);
 	}
 
+	*changed_seqform = false;
+
 	/*
 	 * We must reset log_cnt when isInit or when changing any parameters that
 	 * would affect future nextval allocations.
@@ -1382,14 +1393,19 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 		}
 
 		seqform->seqtypid = newtypid;
+		*changed_seqform = true;
 	}
 	else if (isInit)
+	{
 		seqform->seqtypid = INT8OID;
+		*changed_seqform = true;
+	}
 
 	/* INCREMENT BY */
 	if (increment_by != NULL)
 	{
 		seqform->seqincrement = defGetInt64(increment_by);
+		*changed_seqform = true;
 		if (seqform->seqincrement == 0)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
@@ -1397,22 +1413,30 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 		seqdataform->log_cnt = 0;
 	}
 	else if (isInit)
+	{
 		seqform->seqincrement = 1;
+		*changed_seqform = true;
+	}
 
 	/* CYCLE */
 	if (is_cycled != NULL)
 	{
 		seqform->seqcycle = intVal(is_cycled->arg);
+		*changed_seqform = true;
 		Assert(BoolIsValid(seqform->seqcycle));
 		seqdataform->log_cnt = 0;
 	}
 	else if (isInit)
+	{
 		seqform->seqcycle = false;
+		*changed_seqform = true;
+	}
 
 	/* MAXVALUE (null arg means NO MAXVALUE) */
 	if (max_value != NULL && max_value->arg)
 	{
 		seqform->seqmax = defGetInt64(max_value);
+		*changed_seqform = true;
 		seqdataform->log_cnt = 0;
 	}
 	else if (isInit || max_value != NULL || reset_max_value)
@@ -1429,6 +1453,7 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 		}
 		else
 			seqform->seqmax = -1;	/* descending seq */
+		*changed_seqform = true;
 		seqdataform->log_cnt = 0;
 	}
 
@@ -1450,6 +1475,7 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 	if (min_value != NULL && min_value->arg)
 	{
 		seqform->seqmin = defGetInt64(min_value);
+		*changed_seqform = true;
 		seqdataform->log_cnt = 0;
 	}
 	else if (isInit || min_value != NULL || reset_min_value)
@@ -1466,6 +1492,7 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 		}
 		else
 			seqform->seqmin = 1; /* ascending seq */
+		*changed_seqform = true;
 		seqdataform->log_cnt = 0;
 	}
 
@@ -1499,13 +1526,17 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 
 	/* START WITH */
 	if (start_value != NULL)
+	{
 		seqform->seqstart = defGetInt64(start_value);
+		*changed_seqform = true;
+	}
 	else if (isInit)
 	{
 		if (seqform->seqincrement > 0)
 			seqform->seqstart = seqform->seqmin;	/* ascending seq */
 		else
 			seqform->seqstart = seqform->seqmax;	/* descending seq */
+		*changed_seqform = true;
 	}
 
 	/* crosscheck START */
@@ -1580,6 +1611,7 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 	if (cache_value != NULL)
 	{
 		seqform->seqcache = defGetInt64(cache_value);
+		*changed_seqform = true;
 		if (seqform->seqcache <= 0)
 		{
 			char		buf[100];
@@ -1593,7 +1625,10 @@ init_params(ParseState *pstate, List *options, bool for_identity,
 		seqdataform->log_cnt = 0;
 	}
 	else if (isInit)
+	{
 		seqform->seqcache = 1;
+		*changed_seqform = true;
+	}
 }
 
 /*