Fix ALTER TABLE ADD VIRTUAL GENERATED COLUMN when table rewrite

demo:
CREATE TABLE gtest20a (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) VIRTUAL);
ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT RANDOM() CHECK (b < 60);
ERROR:  no generation expression found for column number 2 of table "pg_temp_17306"

In ATRewriteTable, the variable OIDNewHeap (if valid) corresponding
pg_attrdef default expression entry was not populated.  So OIDNewHeap
cannot be used to call expand_generated_columns_in_expr or
build_generation_expression.  Therefore in ATRewriteTable, we can only
use the existing relation to expand the generated expression.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Srinath Reddy <srinath2133@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CACJufxEJ%3DFoajabWXjszo_yrQeKSxdZ87KJqBW373rSbajKGAA%40mail.gmail.com

src/backend/commands/tablecmds.c

@@ -6137,7 +6137,7 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap)
 		{
 			case CONSTR_CHECK:
 				needscan = true;
-				con->qualstate = ExecPrepareExpr((Expr *) expand_generated_columns_in_expr(con->qual, newrel ? newrel : oldrel, 1), estate);
+				con->qualstate = ExecPrepareExpr((Expr *) expand_generated_columns_in_expr(con->qual, oldrel, 1), estate);
 				break;
 			case CONSTR_FOREIGN:
 				/* Nothing to do here */

src/test/regress/expected/generated_stored.out

@@ -656,6 +656,10 @@ INSERT INTO gtest20a (a) VALUES (10);
 INSERT INTO gtest20a (a) VALUES (30);
 ALTER TABLE gtest20a ADD CHECK (b < 50);  -- fails on existing row
 ERROR:  check constraint "gtest20a_b_check" of relation "gtest20a" is violated by some row
+-- table rewrite cases
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 50); -- fails on existing row
+ERROR:  check constraint "gtest20a_b_check" of relation "gtest20a" is violated by some row
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 61); -- ok
 CREATE TABLE gtest20b (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) STORED);
 INSERT INTO gtest20b (a) VALUES (10);
 INSERT INTO gtest20b (a) VALUES (30);

src/test/regress/expected/generated_virtual.out

@@ -647,6 +647,10 @@ INSERT INTO gtest20a (a) VALUES (10);
 INSERT INTO gtest20a (a) VALUES (30);
 ALTER TABLE gtest20a ADD CHECK (b < 50);  -- fails on existing row
 ERROR:  check constraint "gtest20a_b_check" of relation "gtest20a" is violated by some row
+-- table rewrite cases
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 50); -- fails on existing row
+ERROR:  check constraint "gtest20a_b_check" of relation "gtest20a" is violated by some row
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 61); -- ok
 CREATE TABLE gtest20b (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) VIRTUAL);
 INSERT INTO gtest20b (a) VALUES (10);
 INSERT INTO gtest20b (a) VALUES (30);

src/test/regress/sql/generated_stored.sql

@@ -319,6 +319,9 @@ CREATE TABLE gtest20a (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) STOR
 INSERT INTO gtest20a (a) VALUES (10);
 INSERT INTO gtest20a (a) VALUES (30);
 ALTER TABLE gtest20a ADD CHECK (b < 50);  -- fails on existing row
+-- table rewrite cases
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 50); -- fails on existing row
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 61); -- ok
 
 CREATE TABLE gtest20b (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) STORED);
 INSERT INTO gtest20b (a) VALUES (10);

src/test/regress/sql/generated_virtual.sql

@@ -319,6 +319,9 @@ CREATE TABLE gtest20a (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) VIRT
 INSERT INTO gtest20a (a) VALUES (10);
 INSERT INTO gtest20a (a) VALUES (30);
 ALTER TABLE gtest20a ADD CHECK (b < 50);  -- fails on existing row
+-- table rewrite cases
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 50); -- fails on existing row
+ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT random() CHECK (b < 61); -- ok
 
 CREATE TABLE gtest20b (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) VIRTUAL);
 INSERT INTO gtest20b (a) VALUES (10);