Not drop default col exprs from shard when adding local table to metadata (#6323)

As we did for GENERATED STORED columns in #4613, we should not drop
column
default expressions that are not based on sequences from shard relation
since
such expressions need to exist e.g. for foreign key actions.

For the column default expressions that are based on sequences we cannot
do much, so we need to disallow having ON DELETE SET DEFAULT actions on
such columns in a separate PR, see #6339.

Fixes #6318.

DESCRIPTION: Fixes a bug that might cause inserting incorrect DEFAULT
values when applying foreign key actions

(cherry picked from commit de24a3eda5)

 Conflicts (dropped those changes since pg15 is not supported on 11.0):
	src/test/regress/expected/pg15.out
	src/test/regress/sql/pg15.sql

src/backend/distributed/commands/citus_add_local_table_to_metadata.c

@@ -87,8 +87,8 @@ static List * ReversedOidList(List *oidList);
 static void AppendExplicitIndexIdsToList(Form_pg_index indexForm,
 										 List **explicitIndexIdList,
 										 int flags);
-static void DropDefaultExpressionsAndMoveOwnedSequenceOwnerships(Oid sourceRelationId,
-																 Oid targetRelationId);
+static void DropNextValExprsAndMoveOwnedSeqOwnerships(Oid sourceRelationId,
+													  Oid targetRelationId);
 static void DropDefaultColumnDefinition(Oid relationId, char *columnName);
 static void TransferSequenceOwnership(Oid ownedSequenceId, Oid targetRelationId,
 									  char *columnName);
@@ -363,11 +363,11 @@ CreateCitusLocalTable(Oid relationId, bool cascadeViaForeignKeys, bool autoConve
 
 	/*
 	 * Move sequence ownerships from shard table to shell table and also drop
-	 * DEFAULT expressions from shard relation as we should evaluate such columns
-	 * in shell table when needed.
+	 * DEFAULT expressions based on sequences from shard relation as we should
+	 * evaluate such columns in shell table when needed.
 	 */
-	DropDefaultExpressionsAndMoveOwnedSequenceOwnerships(shardRelationId,
-														 shellRelationId);
+	DropNextValExprsAndMoveOwnedSeqOwnerships(shardRelationId,
+											  shellRelationId);
 
 	InsertMetadataForCitusLocalTable(shellRelationId, shardId, autoConverted);
 
@@ -1153,14 +1153,15 @@ GetRenameStatsCommandList(List *statsOidList, uint64 shardId)
 
 
 /*
- * DropDefaultExpressionsAndMoveOwnedSequenceOwnerships drops default column
- * definitions for relation with sourceRelationId. Also, for each column that
- * defaults to an owned sequence, it grants ownership to the same named column
- * of the relation with targetRelationId.
+ * DropNextValExprsAndMoveOwnedSeqOwnerships drops default column definitions
+ * that are based on sequences for relation with sourceRelationId.
+ *
+ * Also, for each such column that owns a sequence, it grants ownership to the
+ * same named column of the relation with targetRelationId.
  */
 static void
-DropDefaultExpressionsAndMoveOwnedSequenceOwnerships(Oid sourceRelationId,
-													 Oid targetRelationId)
+DropNextValExprsAndMoveOwnedSeqOwnerships(Oid sourceRelationId,
+										  Oid targetRelationId)
 {
 	List *columnNameList = NIL;
 	List *ownedSequenceIdList = NIL;
@@ -1171,9 +1172,28 @@ DropDefaultExpressionsAndMoveOwnedSequenceOwnerships(Oid sourceRelationId,
 	Oid ownedSequenceId = InvalidOid;
 	forboth_ptr_oid(columnName, columnNameList, ownedSequenceId, ownedSequenceIdList)
 	{
-		DropDefaultColumnDefinition(sourceRelationId, columnName);
+		/*
+		 * We drop nextval() expressions because Citus currently evaluates
+		 * nextval() on the shell table, not on the shards. Hence, there is
+		 * no reason for keeping nextval(). Also, distributed/reference table
+		 * shards do not have - so be consistent with those.
+		 *
+		 * Note that we keep other kind of DEFAULT expressions on shards
+		 * because we still want to be able to evaluate DEFAULT expressions
+		 * that are not based on sequences on shards, e.g., for foreign key
+		 * - SET DEFAULT actions.
+		 */
+		AttrNumber columnAttrNumber = get_attnum(sourceRelationId, columnName);
+		if (ColumnDefaultsToNextVal(sourceRelationId, columnAttrNumber))
+		{
+			DropDefaultColumnDefinition(sourceRelationId, columnName);
+		}
 
-		/* column might not own a sequence */
+		/*
+		 * Column might own a sequence without having a nextval() expr on it
+		 * --e.g., due to ALTER SEQUENCE OWNED BY .. --, so check if that is
+		 * the case even if the column doesn't have a DEFAULT.
+		 */
 		if (OidIsValid(ownedSequenceId))
 		{
 			TransferSequenceOwnership(ownedSequenceId, targetRelationId, columnName);

src/backend/distributed/commands/sequence.c

@@ -27,6 +27,7 @@
 #include "nodes/makefuncs.h"
 #include "distributed/worker_create_or_replace.h"
 #include "nodes/parsenodes.h"
+#include "rewrite/rewriteHandler.h"
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
 
@@ -213,6 +214,29 @@ ExtractDefaultColumnsAndOwnedSequences(Oid relationId, List **columnNameList,
 }
 
 
+/*
+ * ColumnDefaultsToNextVal returns true if the column with attrNumber
+ * has a default expression that contains nextval().
+ */
+bool
+ColumnDefaultsToNextVal(Oid relationId, AttrNumber attrNumber)
+{
+	AssertArg(AttributeNumberIsValid(attrNumber));
+
+	Relation relation = RelationIdGetRelation(relationId);
+	Node *defExpr = build_column_default(relation, attrNumber);
+	RelationClose(relation);
+
+	if (defExpr == NULL)
+	{
+		/* column doesn't have a DEFAULT expression */
+		return false;
+	}
+
+	return contain_nextval_expression_walker(defExpr, NULL);
+}
+
+
 /*
  * PreprocessDropSequenceStmt gets called during the planning phase of a DROP SEQUENCE statement
  * and returns a list of DDLJob's that will drop any distributed sequences from the

src/include/distributed/commands/sequence.h

@@ -10,9 +10,11 @@
 #ifndef CITUS_SEQUENCE_H
 #define CITUS_SEQUENCE_H
 
+#include "access/attnum.h"
 #include "nodes/pg_list.h"
 
 
+extern bool ColumnDefaultsToNextVal(Oid relationId, AttrNumber attrNumber);
 extern void ExtractDefaultColumnsAndOwnedSequences(Oid relationId,
 												   List **columnNameList,
 												   List **ownedSequenceIdList);