SQLite

        SELECTTRACE(0x100,pParse,p,("After WHERE-clause push-down:\n"));
        sqlite3TreeViewSelect(0, p, 0);
      }
#endif
    }

    /* Generate code to implement the subquery
    **
    ** The subquery is implemented as a co-routine if all of these are true:
    **   (1)  The subquery is guaranteed to be the outer loop (so that it
    **        does not need to be computed more than once)
    **   (2)  The ALL keyword after SELECT is omitted.  (Applications are
    **        allowed to say "SELECT ALL" instead of just "SELECT" to disable
    **        the use of co-routines.)
    **   (3)  Co-routines are not disabled using sqlite3_test_control()
    **        with SQLITE_TESTCTRL_OPTIMIZATIONS.
    **
    ** TODO: Are there other reasons beside (1) to use a co-routine
    ** implementation?
    */
    if( i==0
     && (pTabList->nSrc==1
            || (pTabList->a[1].fg.jointype&(JT_LEFT|JT_CROSS))!=0)  /* (1) */
     && (p->selFlags & SF_All)==0                                   /* (2) */
     && OptimizationEnabled(db, SQLITE_SubqCoroutine)               /* (3) */
    ){
      /* Implement a co-routine that will return a single row of the result
      ** set on each invocation.
      */
      int addrTop = sqlite3VdbeCurrentAddr(v)+1;
      pItem->regReturn = ++pParse->nMem;
      sqlite3VdbeAddOp3(v, OP_InitCoroutine, pItem->regReturn, 0, addrTop);

#    May you share freely, never taking more than you give.
#
#***********************************************************************
# This file implements regression tests for SQLite library.  The
# focus of this file is testing UNION, INTERSECT and EXCEPT operators
# in SELECT statements.
#


set testdir [file dirname $argv0]
source $testdir/tester.tcl

# Most tests in this file depend on compound-select. But there are a couple
# right at the end that test DISTINCT, so we cannot omit the entire file.
#

   WHERE t0.a=t1.a AND t1.a=33 AND t0.b=456
  UNION
  SELECT DISTINCT t0.id, t0.a, t0.b
    FROM tx AS t0, tx AS t1
   WHERE t0.a=t1.a AND t1.a=33 AND t0.b=789
   ORDER BY 1;
} {1 33 456 2 33 789}

# Enhancement (2016-03-15):  Use a co-routine for subqueries if the
# subquery is guaranteed to be the outer-most query
#
do_execsql_test select4-16.1 {
  DROP TABLE IF EXISTS t1;
  CREATE TABLE t1(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,
  PRIMARY KEY(a,b DESC)) WITHOUT ROWID;

  WITH RECURSIVE c(x) AS (VALUES(1) UNION ALL SELECT x+1 FROM c WHERE x<100)
  INSERT INTO t1(a,b,c,d)
    SELECT x%10, x/10, x, printf('xyz%dabc',x) FROM c;

  SELECT t3.c FROM 
    (SELECT a,max(b) AS m FROM t1 WHERE a>=5 GROUP BY a) AS t2
    JOIN t1 AS t3
  WHERE t2.a=t3.a AND t2.m=t3.b
  ORDER BY t3.a;
} {95 96 97 98 99}
do_execsql_test select4-16.2 {
  SELECT t3.c FROM 
    (SELECT a,max(b) AS m FROM t1 WHERE a>=5 GROUP BY a) AS t2
    CROSS JOIN t1 AS t3
  WHERE t2.a=t3.a AND t2.m=t3.b
  ORDER BY t3.a;
} {95 96 97 98 99}
do_execsql_test select4-16.3 {
  SELECT t3.c FROM 
    (SELECT a,max(b) AS m FROM t1 WHERE a>=5 GROUP BY a) AS t2
    LEFT JOIN t1 AS t3
  WHERE t2.a=t3.a AND t2.m=t3.b
  ORDER BY t3.a;
} {95 96 97 98 99}




finish_test