UNION ALL

[jussisaurio]

Adds support for UNION ALL and introduces Plan::CompoundSelect so that it can be extended to support UNION/EXCEPT/INTERSECT as well

do_execsql_test_on_specific_db {:memory:} select-union-all-1 {
  CREATE TABLE t1(x INTEGER);
  CREATE TABLE t2(x INTEGER); 
  CREATE TABLE t3(x INTEGER);

  INSERT INTO t1 VALUES(1),(2),(3);
  INSERT INTO t2 VALUES(4),(5),(6);
  INSERT INTO t3 VALUES(7),(8),(9);

  SELECT x FROM t1
  UNION ALL 
  SELECT x FROM t2
  UNION ALL
  SELECT x FROM t3;
} {1
2
3
4
5
6
7
8
9}

do_execsql_test_on_specific_db {:memory:} select-union-all-with-filters {
  CREATE TABLE t4(x INTEGER);
  CREATE TABLE t5(x INTEGER);
  CREATE TABLE t6(x INTEGER);

  INSERT INTO t4 VALUES(1),(2),(3),(4);
  INSERT INTO t5 VALUES(5),(6),(7),(8);
  INSERT INTO t6 VALUES(9),(10),(11),(12);

  SELECT x FROM t4 WHERE x > 2
  UNION ALL
  SELECT x FROM t5 WHERE x < 7 
  UNION ALL
  SELECT x FROM t6 WHERE x = 10;
} {3
4
5
6
10}

Supports LIMIT. Currently does not support WITH(), OFFSET or ORDER BY and explicitly returns a parse error if those are present.