SQLite

      exprCodeBetween(pParse, pExpr, target, 0, 0);
      return target;
    }
    case TK_COLLATE: {
      if( !ExprHasProperty(pExpr, EP_Collate) ){
        /* A TK_COLLATE Expr node without the EP_Collate tag is a so-called
        ** "SOFT-COLLATE" that is added to constraints that are pushed down
        ** from outer queries into sub-queries by the WHERE-clause push-down
        ** optimization. Clear subtypes as subtypes may not cross a subquery
        ** boundary.
        */
        assert( pExpr->pLeft );
        sqlite3ExprCode(pParse, pExpr->pLeft, target);
        sqlite3VdbeAddOp1(v, OP_ClrSubtype, target);
        return target;
      }else{
        pExpr = pExpr->pLeft;

** Transformed into:
**
**    SELECT * FROM (SELECT a AS x, c-d AS y FROM t1 WHERE a=5 AND c-d=10)
**     WHERE x=5 AND y=10;
**
** The hope is that the terms added to the inner query will make it more
** efficient.
**
** NAME AMBIGUITY
**
** This optimization is called the "WHERE-clause push-down optimization".
**
** Do not confuse this optimization with another unrelated optimization
** with a similar name:  The "MySQL push-down optimization" causes WHERE
** clause terms that can be evaluated using only the index and without
** reference to the table are run first, so that if they are false,
** unnecessary table seeks are avoided.
**
** RULES
**
** Do not attempt this optimization if:
**
**   (1) (** This restriction was removed on 2017-09-29.  We used to
**           disallow this optimization for aggregate subqueries, but now
**           it is allowed by putting the extra terms on the HAVING clause.
**           The added HAVING clause is pointless if the subquery lacks

**            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 WHERE-clause 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.
**       See also restriction (6) in sqlite3ExprIsSingleTableConstraint().
**
**  (10) The inner query is not the right-hand table of a RIGHT JOIN.
**
**  (11) The subquery is not a VALUES clause
**
**  (12) The WHERE clause is not "rowid ISNULL" or the equivalent.  This
**       case only comes up if SQLite is compiled using

        TREETRACE(0x4000,pParse,p,
            ("After WHERE-clause push-down into subquery %d:\n", pSub->selId));
        sqlite3TreeViewSelect(0, p, 0);
      }
#endif
      assert( pItem->pSelect && (pItem->pSelect->selFlags & SF_PushDown)!=0 );
    }else{
      TREETRACE(0x4000,pParse,p,("WHERE-lcause push-down not possible\n"));
    }

    /* Convert unused result columns of the subquery into simple NULL
    ** expressions, to avoid unneeded searching and computation.
    */
    if( OptimizationEnabled(db, SQLITE_NullUnusedCols)
     && disableUnusedSubqueryResultColumns(pItem)

#define SQLITE_OrderByIdxJoin 0x00000040 /* ORDER BY of joins via index */
#define SQLITE_Transitive     0x00000080 /* Transitive constraints */
#define SQLITE_OmitNoopJoin   0x00000100 /* Omit unused tables in joins */
#define SQLITE_CountOfView    0x00000200 /* The count-of-view optimization */
#define SQLITE_CursorHints    0x00000400 /* Add OP_CursorHint opcodes */
#define SQLITE_Stat4          0x00000800 /* Use STAT4 data */
   /* TH3 expects this value  ^^^^^^^^^^ to be 0x0000800. Don't change it */
#define SQLITE_PushDown       0x00001000 /* WHERE-clause push-down opt */
#define SQLITE_SimplifyJoin   0x00002000 /* Convert LEFT JOIN to JOIN */
#define SQLITE_SkipScan       0x00004000 /* Skip-scans */
#define SQLITE_PropagateConst 0x00008000 /* The constant propagation opt */
#define SQLITE_MinMaxOpt      0x00010000 /* The min/max optimization */
#define SQLITE_SeekScan       0x00020000 /* The OP_SeekScan optimization */
#define SQLITE_OmitOrderBy    0x00040000 /* Omit pointless ORDER BY */
   /* TH3 expects this value  ^^^^^^^^^^ to be 0x40000. Coordinate any change */

#define SF_IncludeHidden 0x0020000 /* Include hidden columns in output */
#define SF_ComplexResult 0x0040000 /* Result contains subquery or function */
#define SF_WhereBegin    0x0080000 /* Really a WhereBegin() call.  Debug Only */
#define SF_WinRewrite    0x0100000 /* Window function rewrite accomplished */
#define SF_View          0x0200000 /* SELECT statement is a view */
#define SF_NoopOrderBy   0x0400000 /* ORDER BY is ignored for this query */
#define SF_UFSrcCheck    0x0800000 /* Check pSrc as required by UPDATE...FROM */
#define SF_PushDown      0x1000000 /* Modified by WHERE-clause push-down opt */
#define SF_MultiPart     0x2000000 /* Has multiple incompatible PARTITIONs */
#define SF_CopyCte       0x4000000 /* SELECT statement is a copy of a CTE */
#define SF_OrderByReqd   0x8000000 /* The ORDER BY clause may not be omitted */
#define SF_UpdateFrom   0x10000000 /* Query originates with UPDATE FROM */
#define SF_Correlated   0x20000000 /* True if references the outer context */

/* True if S exists and has SF_NestedFrom */

  **
  ** iLoop==1: Code only expressions that are entirely covered by pIdx.
  ** iLoop==2: Code remaining expressions that do not contain correlated
  **           sub-queries. 
  ** iLoop==3: Code all remaining expressions.
  **
  ** An effort is made to skip unnecessary iterations of the loop.
  **
  ** This optimization of causing simple query restrictions to occur before
  ** more complex one is call the "push-down" optimization in MySQL.  Here
  ** in SQLite, the name is "MySQL push-down", since there is also another
  ** totally unrelated optimization called "WHERE-clause push-down".
  ** Sometimes the qualifier is omitted, resulting in an ambiguity, so beware.
  */
  iLoop = (pIdx ? 1 : 2);
  do{
    int iNext = 0;                /* Next value for iLoop */
    for(pTerm=pWC->a, j=pWC->nTerm; j>0; j--, pTerm++){
      Expr *pE;
      int skipLikeAddr = 0;

# 2017-04-29
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
#
# Test cases for the push-down optimizations.
#
#
# There are two different meanings for "push-down optimization".
#
#   (1)  "MySQL push-down" means that WHERE clause terms that can be
#        evaluated using only the index and without reference to the
#        table are run first, so that if they are false, unnecessary table
#        seeks are avoided.  See https://sqlite.org/src/info/d7bb79ed3a40419d
#        from 2017-04-29.
#
#   (2)  "WHERE-clause pushdown" means to push WHERE clause terms in
#        outer queries down into subqueries.  See
#        https://sqlite.org/src/info/6df18e949d367629 from 2015-06-02.
#
# This module started out as tests for MySQL push-down only.  But because
# of naming ambiguity, it has picked up test cases for WHERE-clause push-down
# over the years.
#

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

do_execsql_test 1.0 {
  CREATE TABLE t1(a, b, c);

      SELECT x FROM u2 WHERE x=a AND f('two')
    ) AND f('three')=123
  }
  set L
} {three}

# 2022-11-25 dbsqlfuzz crash-3a548de406a50e896c1bf7142692d35d339d697f
# Disable the WHERE-clause push-down optimization for compound subqueries
# if any arm of the compound has an incompatible affinity.
#
reset_db
do_execsql_test 3.1 {
  CREATE TABLE t0(c0 INT);
  INSERT INTO t0 VALUES(0);
  CREATE TABLE t1_a(a INTEGER PRIMARY KEY, b TEXT);
  INSERT INTO t1_a VALUES(1,'one');

     `--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 WHERE-clause 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);

    FROM t1 JOIN t2 ON null RIGHT JOIN t3 ON true
          LEFT JOIN (t4 JOIN t5 ON d+1=e) ON d=4
   WHERE e>0;
} {- - 3 4 5}


# 2024-04-05
# Allow WHERE-clause push-down of operators of the form "expr IN table".
#
reset_db
do_execsql_test 6.0 {
  CREATE TABLE t01(w,x,y,z);
  CREATE TABLE t02(w,x,y,z);
  CREATE VIEW t0(w,x,y,z) AS
    SELECT w,x,y,z FROM t01 UNION ALL SELECT w,x,y,z FROM t02;