SQLite

    pSub = pItem->pSelect;
    if( pSub==0 ) continue;

    /* The code for a subquery should only be generated once, though it is
    ** technically harmless for it to be generated multiple times. The
    ** following assert() will detect if something changes to cause
    ** the same subquery to be coded multiple times, as a signal to the
    ** developers to try to optimize the situation.
    **
    ** Update 2019-07-24:
    ** See ticket https://sqlite.org/src/tktview/c52b09c7f38903b1311cec40.
    ** The dbsqlfuzz fuzzer found a case where the same subquery gets
    ** coded twice.  So this assert() now becomes a testcase().  It should
    ** be very rare, though.
    */
    testcase( pItem->addrFillSub!=0 );

    /* Increment Parse.nHeight by the height of the largest expression
    ** tree referred to by this, the parent select. The child select
    ** may contain expression trees of at most
    ** (SQLITE_MAX_EXPR_DEPTH-Parse.nHeight) height. This is a bit
    ** more conservative than necessary, but much easier than enforcing
    ** an exact limit.

      ** is a register allocated to hold the subroutine return address
      */
      int topAddr;
      int onceAddr = 0;
      int retAddr;
      struct SrcList_item *pPrior;

      testcase( pItem->addrFillSub==0 ); /* Ticket c52b09c7f38903b1311 */
      pItem->regReturn = ++pParse->nMem;
      topAddr = sqlite3VdbeAddOp2(v, OP_Integer, 0, pItem->regReturn);
      pItem->addrFillSub = topAddr+1;
      if( pItem->fg.isCorrelated==0 ){
        /* If the subquery is not correlated and if we are not inside of
        ** a trigger, then we only need to compute the value of the subquery
        ** once. */

do_execsql_test select1-17.2 {
  SELECT * FROM t1,(SELECT * FROM t2 WHERE y=2 ORDER BY y,z LIMIT 4);
} {1 2 3}
do_execsql_test select1-17.3 {
  SELECT * FROM t1,(SELECT * FROM t2 WHERE y=2
         UNION ALL SELECT * FROM t2 WHERE y=3 ORDER BY y,z LIMIT 4);
} {1 2 3}

# 2019-07-24 Ticket https://sqlite.org/src/tktview/c52b09c7f38903b1311
#
do_execsql_test select1-18.1 {
  DROP TABLE IF EXISTS t1;
  DROP TABLE IF EXISTS t2;
  CREATE TABLE t1(c);
  CREATE TABLE t2(x PRIMARY KEY, y);
  INSERT INTO t1(c) VALUES(123);
  INSERT INTO t2(x) VALUES(123);
  SELECT x FROM t2, t1 WHERE x BETWEEN c AND null OR x AND
  x IN ((SELECT x FROM (SELECT x FROM t2, t1 
  WHERE x BETWEEN (SELECT x FROM (SELECT x COLLATE rtrim 
  FROM t2, t1 WHERE x BETWEEN c AND null
  OR x AND x IN (c)), t1 WHERE x BETWEEN c AND null
  OR x AND x IN (c)) AND null
  OR NOT EXISTS(SELECT -4.81 FROM t1, t2 WHERE x BETWEEN c AND null
  OR x AND x IN ((SELECT x FROM (SELECT x FROM t2, t1
  WHERE x BETWEEN (SELECT x FROM (SELECT x BETWEEN c AND null
  OR x AND x IN (c)), t1 WHERE x BETWEEN c AND null
  OR x AND x IN (c)) AND null
  OR x AND x IN (c)), t1 WHERE x BETWEEN c AND null
  OR x AND x IN (c)))) AND x IN (c)
  ), t1 WHERE x BETWEEN c AND null
  OR x AND x IN (c)));
} {}
do_execsql_test select1-18.2 {
  DROP TABLE IF EXISTS t1;
  DROP TABLE IF EXISTS t2;
  CREATE TABLE t1(c);
  CREATE TABLE t2(x PRIMARY KEY, y);
  INSERT INTO t1(c) VALUES(123);
  INSERT INTO t2(x) VALUES(123);
  SELECT x FROM t2, t1 WHERE x BETWEEN c AND (c+1) OR x AND
  x IN ((SELECT x FROM (SELECT x FROM t2, t1 
  WHERE x BETWEEN (SELECT x FROM (SELECT x COLLATE rtrim 
  FROM t2, t1 WHERE x BETWEEN c AND (c+1)
  OR x AND x IN (c)), t1 WHERE x BETWEEN c AND (c+1)
  OR x AND x IN (c)) AND (c+1)
  OR NOT EXISTS(SELECT -4.81 FROM t1, t2 WHERE x BETWEEN c AND (c+1)
  OR x AND x IN ((SELECT x FROM (SELECT x FROM t2, t1
  WHERE x BETWEEN (SELECT x FROM (SELECT x BETWEEN c AND (c+1)
  OR x AND x IN (c)), t1 WHERE x BETWEEN c AND (c+1)
  OR x AND x IN (c)) AND (c+1)
  OR x AND x IN (c)), t1 WHERE x BETWEEN c AND (c+1)
  OR x AND x IN (c)))) AND x IN (c)
  ), t1 WHERE x BETWEEN c AND (c+1)
  OR x AND x IN (c)));
} {123}
do_execsql_test select1-18.3 {
  SELECT 1 FROM t1 WHERE (
    SELECT 2 FROM t2 WHERE (
      SELECT 3 FROM (
        SELECT x FROM t2 WHERE x=c OR x=(SELECT x FROM (VALUES(0)))
      ) WHERE x>c OR x=c
    )
  );
} {1}
do_execsql_test select1-18.4 {
  SELECT 1 FROM t1, t2 WHERE (
    SELECT 3 FROM (
      SELECT x FROM t2 WHERE x=c OR x=(SELECT x FROM (VALUES(0)))
    ) WHERE x>c OR x=c
  );
} {1}

finish_test