SQLite

**       be materialized.  (This restriction is implemented in the calling
**       routine.)
**
**   (8) If the subquery is a compound that uses UNION, INTERSECT,
**       or EXCEPT, then all of the result set columns for all arms of
**       the compound must use the BINARY collating sequence.
**
**   (9) All three of the following are true:
**
**       (9a) The WHERE clause expression originates in the ON or USING clause
**            of a join (either an INNER or an OUTER join), and
**
**       (9b) The subquery is to the right of the ON/USING clause
**
**       (9c) There is a RIGHT JOIN (or FULL JOIN) in between the ON/USING
**            clause and the subquery.
**
**       Without this restriction, the push-down optimization might move
**       the ON/USING filter expression from the left side of a RIGHT JOIN
**       over to the right side, which leads to incorrect answers.
**
**  (10) The inner query is not the right-hand table of a RIGHT JOIN.
**
**  (11) The subquery is not a VALUES clause
**
** Return 0 if no changes are made and non-zero if one or more WHERE clause
** terms are duplicated into the subquery.
*/
static int pushDownWhereTerms(
  Parse *pParse,        /* Parse context (for malloc() and error reporting) */
  Select *pSubq,        /* The subquery whose WHERE clause is to be augmented */
  Expr *pWhere,         /* The WHERE clause of the outer query */
  SrcList *pSrcList,    /* The complete from clause of the outer query */
  int iSrc              /* Which FROM clause term to try to push into  */
){
  Expr *pNew;
  SrcItem *pSrc;        /* The subquery FROM term into which WHERE is pushed */
  int nChng = 0;
  pSrc = &pSrcList->a[iSrc];
  if( pWhere==0 ) return 0;
  if( pSubq->selFlags & (SF_Recursive|SF_MultiPart) ){
    return 0;           /* restrictions (2) and (11) */
  }
  if( pSrc->fg.jointype & (JT_LTORJ|JT_RIGHT) ){
    return 0;           /* restrictions (10) */
  }

  if( pSubq->pPrior ){
    Select *pSel;
    int notUnionAll = 0;
    for(pSel=pSubq; pSel; pSel=pSel->pPrior){
      u8 op = pSel->op;
      assert( op==TK_ALL || op==TK_SELECT 

  }
#endif

  if( pSubq->pLimit!=0 ){
    return 0; /* restriction (3) */
  }
  while( pWhere->op==TK_AND ){
    nChng += pushDownWhereTerms(pParse, pSubq, pWhere->pRight, pSrcList, iSrc);
    pWhere = pWhere->pLeft;
  }

  if( ExprHasProperty(pWhere, EP_OuterON|EP_InnerON) /* (9a) */
   && (pSrcList->a[0].fg.jointype & JT_LTORJ)!=0     /* Fast pre-test of (9c) */
  ){
    int jj;
    for(jj=0; jj<iSrc; jj++){
      if( pWhere->w.iJoin==pSrcList->a[jj].iCursor ){
        /* If we reach this point, both (9a) and (9b) are satisfied.
        ** The following loop checks (9c):
        */
        for(jj++; jj<iSrc; jj++){
          if( (pSrcList->a[jj].fg.jointype & JT_RIGHT)!=0 ){
            return 0;  /* restriction (9) */
          }
        }
      }
    }
  }

#if 0  /* Legacy code. Checks now done by sqlite3ExprIsTableConstraint() */
  if( isLeftJoin
   && (ExprHasProperty(pWhere,EP_OuterON)==0
         || pWhere->w.iJoin!=iCursor)
  ){
    return 0; /* restriction (4) */

    /* Make copies of constant WHERE-clause terms in the outer query down
    ** inside the subquery.  This can help the subquery to run more efficiently.
    */
    if( OptimizationEnabled(db, SQLITE_PushDown)
     && (pItem->fg.isCte==0 
         || (pItem->u2.pCteUse->eM10d!=M10d_Yes && pItem->u2.pCteUse->nUse<2))
     && pushDownWhereTerms(pParse, pSub, p->pWhere, pTabList, i)
    ){
#if TREETRACE_ENABLED
      if( sqlite3TreeTrace & 0x4000 ){
        TREETRACE(0x4000,pParse,p,
            ("After WHERE-clause push-down into subquery %d:\n", pSub->selId));
        sqlite3TreeViewSelect(0, p, 0);
      }

  |  `--SCAN t1
  `--SCALAR SUBQUERY xxxxxx
     `--SCAN t2
}
# ^^^^^^^^^^^^^^^^^^^^
# The query should be converted into:
#   SELECT (SELECT count(*) FROM t1)+(SELECT count(*) FROM t2)

# 2023-05-09 https://sqlite.org/forum/forumpost/a7d4be7fb6
# Restriction (9) on the push-down optimization.
# 
reset_db
db null -
do_execsql_test 4.1 {
  CREATE TABLE t1(a INT);
  CREATE TABLE t2(b INT);
  CREATE TABLE t3(c INT);
  INSERT INTO t3(c) VALUES(3);
  CREATE TABLE t4(d INT);
  CREATE TABLE t5(e INT);
  INSERT INTO t5(e) VALUES(5);
  CREATE VIEW v6(f,g) AS SELECT d, e FROM t4 RIGHT JOIN t5 ON true;
  SELECT * FROM  t1 JOIN t2 ON false RIGHT JOIN t3 ON true CROSS JOIN v6;
} {- - 3 - 5}
do_execsql_test 4.2 {
  SELECT * FROM v6 JOIN t5 ON false RIGHT JOIN t3 ON true;
} {- - - 3}
do_execsql_test 4.3 {
  SELECT * FROM t1 JOIN t2 ON false JOIN v6 ON true RIGHT JOIN t3 ON true;
} {- - - - 3}

finish_test