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

src/test/regress/expected/generated.out

@@ -219,12 +219,54 @@ SELECT * FROM gtest1;
  4 | 8
 (2 rows)
 
--- test inheritance mismatch
+CREATE TABLE gtest_normal (a int, b int);
+CREATE TABLE gtest_normal_child (a int, b int GENERATED ALWAYS AS (a * 2) STORED) INHERITS (gtest_normal);
+NOTICE:  merging column "a" with inherited definition
+NOTICE:  merging column "b" with inherited definition
+\d gtest_normal_child
+                      Table "public.gtest_normal_child"
+ Column |  Type   | Collation | Nullable |              Default               
+--------+---------+-----------+----------+------------------------------------
+ a      | integer |           |          | 
+ b      | integer |           |          | generated always as (a * 2) stored
+Inherits: gtest_normal
+
+INSERT INTO gtest_normal (a) VALUES (1);
+INSERT INTO gtest_normal_child (a) VALUES (2);
+SELECT * FROM gtest_normal;
+ a | b 
+---+---
+ 1 |  
+ 2 | 4
+(2 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
+ERROR:  child column "b" specifies generation expression
+HINT:  Omit the generation expression in the definition of the child table column to inherit the generation expression from the parent table.
+CREATE TABLE gtestx (x int, b int DEFAULT 10) INHERITS (gtest1);  -- error
+NOTICE:  merging column "b" with inherited definition
+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
+-- test multiple inheritance mismatches
 CREATE TABLE gtesty (x int, b int);
 CREATE TABLE gtest1_2 () INHERITS (gtest1, gtesty);  -- error
 NOTICE:  merging multiple inherited definitions of column "b"
 ERROR:  inherited column "b" has a generation conflict
 DROP TABLE gtesty;
+CREATE TABLE gtesty (x int, b int GENERATED ALWAYS AS (x * 22) STORED);
+CREATE TABLE gtest1_2 () INHERITS (gtest1, gtesty);  -- error
+NOTICE:  merging multiple inherited definitions of column "b"
+ERROR:  column "b" inherits conflicting generation expressions
+DROP TABLE gtesty;
+CREATE TABLE gtesty (x int, b int DEFAULT 55);
+CREATE TABLE gtest1_2 () INHERITS (gtest0, gtesty);  -- error
+NOTICE:  merging multiple inherited definitions of column "b"
+ERROR:  inherited column "b" has a generation conflict
+DROP TABLE gtesty;
 -- test stored update
 CREATE TABLE gtest3 (a int, b int GENERATED ALWAYS AS (a * 3) STORED);
 INSERT INTO gtest3 (a) VALUES (1), (2), (3), (NULL);

src/test/regress/sql/generated.sql

@@ -89,11 +89,31 @@ INSERT INTO gtest1_1 VALUES (4);
 SELECT * FROM gtest1_1;
 SELECT * FROM gtest1;
 
--- test inheritance mismatch
+CREATE TABLE gtest_normal (a int, b int);
+CREATE TABLE gtest_normal_child (a int, b int GENERATED ALWAYS AS (a * 2) STORED) INHERITS (gtest_normal);
+\d gtest_normal_child
+INSERT INTO gtest_normal (a) VALUES (1);
+INSERT INTO gtest_normal_child (a) VALUES (2);
+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
+
+-- test multiple inheritance mismatches
 CREATE TABLE gtesty (x int, b int);
 CREATE TABLE gtest1_2 () INHERITS (gtest1, gtesty);  -- error
 DROP TABLE gtesty;
 
+CREATE TABLE gtesty (x int, b int GENERATED ALWAYS AS (x * 22) STORED);
+CREATE TABLE gtest1_2 () INHERITS (gtest1, gtesty);  -- error
+DROP TABLE gtesty;
+
+CREATE TABLE gtesty (x int, b int DEFAULT 55);
+CREATE TABLE gtest1_2 () INHERITS (gtest0, gtesty);  -- error
+DROP TABLE gtesty;
+
 -- test stored update
 CREATE TABLE gtest3 (a int, b int GENERATED ALWAYS AS (a * 3) STORED);
 INSERT INTO gtest3 (a) VALUES (1), (2), (3), (NULL);