Fix ALTER TABLE / INHERIT with generated columns

When running ALTER TABLE t2 INHERIT t1, we must check that columns in
t2 that correspond to a generated column in t1 are also generated and
have the same generation expression.  Otherwise, this would allow
creating setups that a normal CREATE TABLE sequence would not allow.

Discussion: https://www.postgresql.org/message-id/22de27f6-7096-8d96-4619-7b882932ca25@2ndquadrant.com

src/backend/commands/tablecmds.c

@@ -13909,6 +13909,66 @@ MergeAttributesIntoExisting(Relation child_rel, Relation parent_rel)
 						 errmsg("column \"%s\" in child table must be marked NOT NULL",
 								attributeName)));
 
+			/*
+			 * If parent column is generated, child column must be, too.
+			 */
+			if (attribute->attgenerated && !childatt->attgenerated)
+				ereport(ERROR,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("column \"%s\" in child table must be a generated column",
+								attributeName)));
+
+			/*
+			 * Check that both generation expressions match.
+			 *
+			 * The test we apply is to see whether they reverse-compile to the
+			 * same source string.  This insulates us from issues like whether
+			 * attributes have the same physical column numbers in parent and
+			 * child relations.  (See also constraints_equivalent().)
+			 */
+			if (attribute->attgenerated && childatt->attgenerated)
+			{
+				TupleConstr *child_constr = child_rel->rd_att->constr;
+				TupleConstr *parent_constr = parent_rel->rd_att->constr;
+				char	   *child_expr = NULL;
+				char	   *parent_expr = NULL;
+
+				Assert(child_constr != NULL);
+				Assert(parent_constr != NULL);
+
+				for (int i = 0; i < child_constr->num_defval; i++)
+				{
+					if (child_constr->defval[i].adnum == childatt->attnum)
+					{
+						child_expr =
+							TextDatumGetCString(DirectFunctionCall2(pg_get_expr,
+																	CStringGetTextDatum(child_constr->defval[i].adbin),
+																	ObjectIdGetDatum(child_rel->rd_id)));
+						break;
+					}
+				}
+				Assert(child_expr != NULL);
+
+				for (int i = 0; i < parent_constr->num_defval; i++)
+				{
+					if (parent_constr->defval[i].adnum == attribute->attnum)
+					{
+						parent_expr =
+							TextDatumGetCString(DirectFunctionCall2(pg_get_expr,
+																	CStringGetTextDatum(parent_constr->defval[i].adbin),
+																	ObjectIdGetDatum(parent_rel->rd_id)));
+						break;
+					}
+				}
+				Assert(parent_expr != NULL);
+
+				if (strcmp(child_expr, parent_expr) != 0)
+					ereport(ERROR,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("column \"%s\" in child table has a conflicting generation expression",
+								attributeName)));
+			}
+
 			/*
 			 * OK, bump the child column's inheritance count.  (If we fail
 			 * later on, this change will just roll back.)

src/test/regress/expected/generated.out

@@ -240,6 +240,17 @@ SELECT * FROM gtest_normal;
  2 | 4
 (2 rows)
 
+CREATE TABLE gtest_normal_child2 (a int, b int GENERATED ALWAYS AS (a * 3) STORED);
+ALTER TABLE gtest_normal_child2 INHERIT gtest_normal;
+INSERT INTO gtest_normal_child2 (a) VALUES (3);
+SELECT * FROM gtest_normal;
+ a | b 
+---+---
+ 1 |  
+ 2 | 4
+ 3 | 9
+(3 rows)
+
 -- test inheritance mismatches between parent and child
 CREATE TABLE gtestx (x int, b int GENERATED ALWAYS AS (a * 22) STORED) INHERITS (gtest1);  -- error
 NOTICE:  merging column "b" with inherited definition
@@ -251,6 +262,16 @@ ERROR:  column "b" inherits from generated column but specifies default
 CREATE TABLE gtestx (x int, b int GENERATED ALWAYS AS IDENTITY) INHERITS (gtest1);  -- error
 NOTICE:  merging column "b" with inherited definition
 ERROR:  column "b" inherits from generated column but specifies identity
+CREATE TABLE gtestxx_1 (a int NOT NULL, b int);
+ALTER TABLE gtestxx_1 INHERIT gtest1;  -- error
+ERROR:  column "b" in child table must be a generated column
+CREATE TABLE gtestxx_2 (a int NOT NULL, b int GENERATED ALWAYS AS (a * 22) STORED);
+ALTER TABLE gtestxx_2 INHERIT gtest1;  -- error
+ERROR:  column "b" in child table has a conflicting generation expression
+CREATE TABLE gtestxx_3 (a int NOT NULL, b int GENERATED ALWAYS AS (a * 2) STORED);
+ALTER TABLE gtestxx_3 INHERIT gtest1;  -- ok
+CREATE TABLE gtestxx_4 (b int GENERATED ALWAYS AS (a * 2) STORED, a int NOT NULL);
+ALTER TABLE gtestxx_4 INHERIT gtest1;  -- ok
 -- test multiple inheritance mismatches
 CREATE TABLE gtesty (x int, b int);
 CREATE TABLE gtest1_2 () INHERITS (gtest1, gtesty);  -- error

src/test/regress/sql/generated.sql

@@ -96,11 +96,25 @@ INSERT INTO gtest_normal (a) VALUES (1);
 INSERT INTO gtest_normal_child (a) VALUES (2);
 SELECT * FROM gtest_normal;
 
+CREATE TABLE gtest_normal_child2 (a int, b int GENERATED ALWAYS AS (a * 3) STORED);
+ALTER TABLE gtest_normal_child2 INHERIT gtest_normal;
+INSERT INTO gtest_normal_child2 (a) VALUES (3);
+SELECT * FROM gtest_normal;
+
 -- test inheritance mismatches between parent and child
 CREATE TABLE gtestx (x int, b int GENERATED ALWAYS AS (a * 22) STORED) INHERITS (gtest1);  -- error
 CREATE TABLE gtestx (x int, b int DEFAULT 10) INHERITS (gtest1);  -- error
 CREATE TABLE gtestx (x int, b int GENERATED ALWAYS AS IDENTITY) INHERITS (gtest1);  -- error
 
+CREATE TABLE gtestxx_1 (a int NOT NULL, b int);
+ALTER TABLE gtestxx_1 INHERIT gtest1;  -- error
+CREATE TABLE gtestxx_2 (a int NOT NULL, b int GENERATED ALWAYS AS (a * 22) STORED);
+ALTER TABLE gtestxx_2 INHERIT gtest1;  -- error
+CREATE TABLE gtestxx_3 (a int NOT NULL, b int GENERATED ALWAYS AS (a * 2) STORED);
+ALTER TABLE gtestxx_3 INHERIT gtest1;  -- ok
+CREATE TABLE gtestxx_4 (b int GENERATED ALWAYS AS (a * 2) STORED, a int NOT NULL);
+ALTER TABLE gtestxx_4 INHERIT gtest1;  -- ok
+
 -- test multiple inheritance mismatches
 CREATE TABLE gtesty (x int, b int);
 CREATE TABLE gtest1_2 () INHERITS (gtest1, gtesty);  -- error