Fix several DDL issues of generated columns versus inheritance

Several combinations of generated columns and inheritance in CREATE
TABLE were not handled correctly.  Specifically:

- Disallow a child column specifying a generation expression if the
  parent column is a generated column.  The child column definition
  must be unadorned and the parent column's generation expression will
  be copied.

- Prohibit a child column of a generated parent column specifying
  default values or identity.

- Allow a child column of a not-generated parent column specifying
  itself as a generated column.  This previously did not work, but it
  was possible to arrive at the state via other means (involving ALTER
  TABLE), so it seems sensible to support it.

Add tests for each case.  Also add documentation about the rules
involving generated columns and inheritance.

Discussion:
    https://www.postgresql.org/message-id/flat/15830.1575468847%40sss.pgh.pa.us
    https://www.postgresql.org/message-id/flat/2678bad1-048f-519a-ef24-b12962f41807%40enterprisedb.com
    https://www.postgresql.org/message-id/flat/CAJvUf_u4h0DxkCMCeEKAWCuzGUTnDP-G5iVmSwxLQSXn0_FWNQ%40mail.gmail.com

1 files changed, 56 insertions, 5 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 2fba83504b6..a518b552b3d 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -2618,12 +2618,55 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 				def->is_local = true;
 				/* Merge of NOT NULL constraints = OR 'em together */
 				def->is_not_null |= newdef->is_not_null;
+
+				/*
+				 * Check for conflicts related to generated columns.
+				 *
+				 * If the parent column is generated, the child column must be
+				 * unadorned and will be made a generated column.  (We could
+				 * in theory allow the child column definition specifying the
+				 * exact same generation expression, but that's a bit
+				 * complicated to implement and doesn't seem very useful.)  We
+				 * also check that the child column doesn't specify a default
+				 * value or identity, which matches the rules for a single
+				 * column in parse_util.c.
+				 */
+				if (def->generated)
+				{
+					if (newdef->generated)
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_COLUMN_DEFINITION),
+								 errmsg("child column \"%s\" specifies generation expression",
+										def->colname),
+								 errhint("Omit the generation expression in the definition of the child table column to inherit the generation expression from the parent table.")));
+					if (newdef->raw_default && !newdef->generated)
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_COLUMN_DEFINITION),
+								 errmsg("column \"%s\" inherits from generated column but specifies default",
+										def->colname)));
+					if (newdef->identity)
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_COLUMN_DEFINITION),
+								 errmsg("column \"%s\" inherits from generated column but specifies identity",
+										def->colname)));
+				}
+				/*
+				 * If the parent column is not generated, then take whatever
+				 * the child column definition says.
+				 */
+				else
+				{
+					if (newdef->generated)
+						def->generated = newdef->generated;
+				}
+
 				/* If new def has a default, override previous default */
 				if (newdef->raw_default != NULL)
 				{
 					def->raw_default = newdef->raw_default;
 					def->cooked_default = newdef->cooked_default;
 				}
+
 			}
 			else
 			{
@@ -2709,11 +2752,19 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 			ColumnDef  *def = lfirst(entry);
 
 			if (def->cooked_default == &bogus_marker)
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_COLUMN_DEFINITION),
-						 errmsg("column \"%s\" inherits conflicting default values",
-								def->colname),
-						 errhint("To resolve the conflict, specify a default explicitly.")));
+			{
+				if (def->generated)
+					ereport(ERROR,
+							(errcode(ERRCODE_INVALID_COLUMN_DEFINITION),
+							 errmsg("column \"%s\" inherits conflicting generation expressions",
+									def->colname)));
+				else
+					ereport(ERROR,
+							(errcode(ERRCODE_INVALID_COLUMN_DEFINITION),
+							 errmsg("column \"%s\" inherits conflicting default values",
+									def->colname),
+							 errhint("To resolve the conflict, specify a default explicitly.")));
+			}
 		}
 	}