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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

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Overview
Comment:Move terms of the HAVING clause that reference only columns in the GROUP BY clause over to the WHERE clause, resulting in a faster query plan.
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Files: files | file ages | folders
SHA3-256: 47cbb471d056c8e1834a5ca72491404a3bfb273b5ff7bdd84b98d263938ea874
User & Date: drh 2017-05-02 16:55:07
Context
2017-05-02
17:54
Reuse the same materialization of a view when that view appears in a query more than once, such as in a self-join. check-in: 9e35c89d user: drh tags: trunk
16:55
Move terms of the HAVING clause that reference only columns in the GROUP BY clause over to the WHERE clause, resulting in a faster query plan. check-in: 47cbb471 user: drh tags: trunk
16:46
Additional comments on the sqlite3ExprIsConstantOrGroupBy() routine. No code changes. Closed-Leaf check-in: 8424492e user: drh tags: having-where-optimization
01:30
Fix typo in comment. No changes to code. check-in: 6674814a user: mistachkin tags: trunk
Changes
Hide Diffs Unified Diffs Ignore Whitespace Patch

Changes to src/expr.c.

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** expression must not refer to any non-deterministic function nor any
** table other than iCur.
*/
int sqlite3ExprIsTableConstant(Expr *p, int iCur){
  return exprIsConst(p, 3, iCur);
}




























































/*
** Walk an expression tree.  Return non-zero if the expression is constant
** or a function call with constant arguments.  Return and 0 if there
** are any variables.
**
** For the purposes of this function, a double-quoted string (ex: "abc")
** is considered a variable but a single-quoted string (ex: 'abc') is







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** expression must not refer to any non-deterministic function nor any
** table other than iCur.
*/
int sqlite3ExprIsTableConstant(Expr *p, int iCur){
  return exprIsConst(p, 3, iCur);
}


/*
** sqlite3WalkExpr() callback used by sqlite3ExprIsConstantOrGroupBy().
*/
static int exprNodeIsConstantOrGroupBy(Walker *pWalker, Expr *pExpr){
  ExprList *pGroupBy = pWalker->u.pGroupBy;
  int i;

  /* Check if pExpr is identical to any GROUP BY term. If so, consider
  ** it constant.  */
  for(i=0; i<pGroupBy->nExpr; i++){
    Expr *p = pGroupBy->a[i].pExpr;
    if( sqlite3ExprCompare(pExpr, p, -1)<2 ){
      CollSeq *pColl = sqlite3ExprCollSeq(pWalker->pParse, p);
      if( pColl==0 || sqlite3_stricmp("BINARY", pColl->zName)==0 ){
        return WRC_Prune;
      }
    }
  }

  /* Check if pExpr is a sub-select. If so, consider it variable. */
  if( ExprHasProperty(pExpr, EP_xIsSelect) ){
    pWalker->eCode = 0;
    return WRC_Abort;
  }

  return exprNodeIsConstant(pWalker, pExpr);
}

/*
** Walk the expression tree passed as the first argument. Return non-zero
** if the expression consists entirely of constants or copies of terms 
** in pGroupBy that sort with the BINARY collation sequence.
**
** This routine is used to determine if a term of the HAVING clause can
** be promoted into the WHERE clause.  In order for such a promotion to work,
** the value of the HAVING clause term must be the same for all members of
** a "group".  The requirement that the GROUP BY term must be BINARY
** assumes that no other collating sequence will have a finer-grained
** grouping than binary.  In other words (A=B COLLATE binary) implies
** A=B in every other collating sequence.  The requirement that the
** GROUP BY be BINARY is stricter than necessary.  It would also work
** to promote HAVING clauses that use the same alternative collating
** sequence as the GROUP BY term, but that is much harder to check,
** alternative collating sequences are uncommon, and this is only an
** optimization, so we take the easy way out and simply require the
** GROUP BY to use the BINARY collating sequence.
*/
int sqlite3ExprIsConstantOrGroupBy(Parse *pParse, Expr *p, ExprList *pGroupBy){
  Walker w;
  memset(&w, 0, sizeof(w));
  w.eCode = 1;
  w.xExprCallback = exprNodeIsConstantOrGroupBy;
  w.u.pGroupBy = pGroupBy;
  w.pParse = pParse;
  sqlite3WalkExpr(&w, p);
  return w.eCode;
}

/*
** Walk an expression tree.  Return non-zero if the expression is constant
** or a function call with constant arguments.  Return and 0 if there
** are any variables.
**
** For the purposes of this function, a double-quoted string (ex: "abc")
** is considered a variable but a single-quoted string (ex: 'abc') is

Changes to src/select.c.

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        pParse->pVdbe, OP_Explain, pParse->iSelectId, 0, 0, zEqp, P4_DYNAMIC
    );
  }
}
#else
# define explainSimpleCount(a,b,c)
#endif








































































/*
** Generate code for the SELECT statement given in the p argument.  
**
** The results are returned according to the SelectDest structure.
** See comments in sqliteInt.h for further information.
**
................................................................................
    sNC.pAggInfo = &sAggInfo;
    sAggInfo.mnReg = pParse->nMem+1;
    sAggInfo.nSortingColumn = pGroupBy ? pGroupBy->nExpr : 0;
    sAggInfo.pGroupBy = pGroupBy;
    sqlite3ExprAnalyzeAggList(&sNC, pEList);
    sqlite3ExprAnalyzeAggList(&sNC, sSort.pOrderBy);
    if( pHaving ){





      sqlite3ExprAnalyzeAggregates(&sNC, pHaving);
    }
    sAggInfo.nAccumulator = sAggInfo.nColumn;
    for(i=0; i<sAggInfo.nFunc; i++){
      assert( !ExprHasProperty(sAggInfo.aFunc[i].pExpr, EP_xIsSelect) );
      sNC.ncFlags |= NC_InAggFunc;
      sqlite3ExprAnalyzeAggList(&sNC, sAggInfo.aFunc[i].pExpr->x.pList);







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        pParse->pVdbe, OP_Explain, pParse->iSelectId, 0, 0, zEqp, P4_DYNAMIC
    );
  }
}
#else
# define explainSimpleCount(a,b,c)
#endif

/*
** Context object for havingToWhereExprCb().
*/
struct HavingToWhereCtx {
  Expr **ppWhere;
  ExprList *pGroupBy;
};

/*
** sqlite3WalkExpr() callback used by havingToWhere().
**
** If the node passed to the callback is a TK_AND node, return 
** WRC_Continue to tell sqlite3WalkExpr() to iterate through child nodes.
**
** Otherwise, return WRC_Prune. In this case, also check if the 
** sub-expression matches the criteria for being moved to the WHERE
** clause. If so, add it to the WHERE clause and replace the sub-expression
** within the HAVING expression with a constant "1".
*/
static int havingToWhereExprCb(Walker *pWalker, Expr *pExpr){
  if( pExpr->op!=TK_AND ){
    struct HavingToWhereCtx *p = pWalker->u.pHavingCtx;
    if( sqlite3ExprIsConstantOrGroupBy(pWalker->pParse, pExpr, p->pGroupBy) ){
      sqlite3 *db = pWalker->pParse->db;
      Expr *pNew = sqlite3ExprAlloc(db, TK_INTEGER, &sqlite3IntTokens[1], 0);
      if( pNew ){
        Expr *pWhere = *(p->ppWhere);
        SWAP(Expr, *pNew, *pExpr);
        pNew = sqlite3ExprAnd(db, pWhere, pNew);
        *(p->ppWhere) = pNew;
      }
    }
    return WRC_Prune;
  }
  return WRC_Continue;
}

/*
** Transfer eligible terms from the HAVING clause of a query, which is
** processed after grouping, to the WHERE clause, which is processed before
** grouping. For example, the query:
**
**   SELECT * FROM <tables> WHERE a=? GROUP BY b HAVING b=? AND c=?
**
** can be rewritten as:
**
**   SELECT * FROM <tables> WHERE a=? AND b=? GROUP BY b HAVING c=?
**
** A term of the HAVING expression is eligible for transfer if it consists
** entirely of constants and expressions that are also GROUP BY terms that
** use the "BINARY" collation sequence.
*/
static void havingToWhere(
  Parse *pParse,
  ExprList *pGroupBy,
  Expr *pHaving, 
  Expr **ppWhere
){
  struct HavingToWhereCtx sCtx;
  Walker sWalker;

  sCtx.ppWhere = ppWhere;
  sCtx.pGroupBy = pGroupBy;

  memset(&sWalker, 0, sizeof(sWalker));
  sWalker.pParse = pParse;
  sWalker.xExprCallback = havingToWhereExprCb;
  sWalker.u.pHavingCtx = &sCtx;
  sqlite3WalkExpr(&sWalker, pHaving);
}

/*
** Generate code for the SELECT statement given in the p argument.  
**
** The results are returned according to the SelectDest structure.
** See comments in sqliteInt.h for further information.
**
................................................................................
    sNC.pAggInfo = &sAggInfo;
    sAggInfo.mnReg = pParse->nMem+1;
    sAggInfo.nSortingColumn = pGroupBy ? pGroupBy->nExpr : 0;
    sAggInfo.pGroupBy = pGroupBy;
    sqlite3ExprAnalyzeAggList(&sNC, pEList);
    sqlite3ExprAnalyzeAggList(&sNC, sSort.pOrderBy);
    if( pHaving ){
      if( pGroupBy ){
        assert( pWhere==p->pWhere );
        havingToWhere(pParse, pGroupBy, pHaving, &p->pWhere);
        pWhere = p->pWhere;
      }
      sqlite3ExprAnalyzeAggregates(&sNC, pHaving);
    }
    sAggInfo.nAccumulator = sAggInfo.nColumn;
    for(i=0; i<sAggInfo.nFunc; i++){
      assert( !ExprHasProperty(sAggInfo.aFunc[i].pExpr, EP_xIsSelect) );
      sNC.ncFlags |= NC_InAggFunc;
      sqlite3ExprAnalyzeAggList(&sNC, sAggInfo.aFunc[i].pExpr->x.pList);

Changes to src/sqliteInt.h.

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  Parse *pParse;                            /* Parser context.  */
  int (*xExprCallback)(Walker*, Expr*);     /* Callback for expressions */
  int (*xSelectCallback)(Walker*,Select*);  /* Callback for SELECTs */
  void (*xSelectCallback2)(Walker*,Select*);/* Second callback for SELECTs */
  int walkerDepth;                          /* Number of subqueries */
  u8 eCode;                                 /* A small processing code */
  union {                                   /* Extra data for callback */
    NameContext *pNC;                          /* Naming context */
    int n;                                     /* A counter */
    int iCur;                                  /* A cursor number */
    SrcList *pSrcList;                         /* FROM clause */
    struct SrcCount *pSrcCount;                /* Counting column references */
    struct CCurHint *pCCurHint;                /* Used by codeCursorHint() */
    int *aiCol;                                /* array of column indexes */
    struct IdxCover *pIdxCover;                /* Check for index coverage */
    struct IdxExprTrans *pIdxTrans;            /* Convert indexed expr to column */


  } u;
};

/* Forward declarations */
int sqlite3WalkExpr(Walker*, Expr*);
int sqlite3WalkExprList(Walker*, ExprList*);
int sqlite3WalkSelect(Walker*, Select*);
................................................................................
void sqlite3RollbackTransaction(Parse*);
void sqlite3Savepoint(Parse*, int, Token*);
void sqlite3CloseSavepoints(sqlite3 *);
void sqlite3LeaveMutexAndCloseZombie(sqlite3*);
int sqlite3ExprIsConstant(Expr*);
int sqlite3ExprIsConstantNotJoin(Expr*);
int sqlite3ExprIsConstantOrFunction(Expr*, u8);

int sqlite3ExprIsTableConstant(Expr*,int);
#ifdef SQLITE_ENABLE_CURSOR_HINTS
int sqlite3ExprContainsSubquery(Expr*);
#endif
int sqlite3ExprIsInteger(Expr*, int*);
int sqlite3ExprCanBeNull(const Expr*);
int sqlite3ExprNeedsNoAffinityChange(const Expr*, char);







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  Parse *pParse;                            /* Parser context.  */
  int (*xExprCallback)(Walker*, Expr*);     /* Callback for expressions */
  int (*xSelectCallback)(Walker*,Select*);  /* Callback for SELECTs */
  void (*xSelectCallback2)(Walker*,Select*);/* Second callback for SELECTs */
  int walkerDepth;                          /* Number of subqueries */
  u8 eCode;                                 /* A small processing code */
  union {                                   /* Extra data for callback */
    NameContext *pNC;                         /* Naming context */
    int n;                                    /* A counter */
    int iCur;                                 /* A cursor number */
    SrcList *pSrcList;                        /* FROM clause */
    struct SrcCount *pSrcCount;               /* Counting column references */
    struct CCurHint *pCCurHint;               /* Used by codeCursorHint() */
    int *aiCol;                               /* array of column indexes */
    struct IdxCover *pIdxCover;               /* Check for index coverage */
    struct IdxExprTrans *pIdxTrans;           /* Convert indexed expr to column */
    ExprList *pGroupBy;                       /* GROUP BY clause */
    struct HavingToWhereCtx *pHavingCtx;      /* HAVING to WHERE clause ctx */
  } u;
};

/* Forward declarations */
int sqlite3WalkExpr(Walker*, Expr*);
int sqlite3WalkExprList(Walker*, ExprList*);
int sqlite3WalkSelect(Walker*, Select*);
................................................................................
void sqlite3RollbackTransaction(Parse*);
void sqlite3Savepoint(Parse*, int, Token*);
void sqlite3CloseSavepoints(sqlite3 *);
void sqlite3LeaveMutexAndCloseZombie(sqlite3*);
int sqlite3ExprIsConstant(Expr*);
int sqlite3ExprIsConstantNotJoin(Expr*);
int sqlite3ExprIsConstantOrFunction(Expr*, u8);
int sqlite3ExprIsConstantOrGroupBy(Parse*, Expr*, ExprList*);
int sqlite3ExprIsTableConstant(Expr*,int);
#ifdef SQLITE_ENABLE_CURSOR_HINTS
int sqlite3ExprContainsSubquery(Expr*);
#endif
int sqlite3ExprIsInteger(Expr*, int*);
int sqlite3ExprCanBeNull(const Expr*);
int sqlite3ExprNeedsNoAffinityChange(const Expr*, char);

Added test/having.test.





















































































































































































































































































































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# 2017 April 30
#
# 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 the HAVING->WHERE optimization.
#

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

do_execsql_test 1.0 {
  CREATE TABLE t2(c, d);

  CREATE TABLE t1(a, b);
  INSERT INTO t1 VALUES(1, 1);
  INSERT INTO t1 VALUES(2, 2);
  INSERT INTO t1 VALUES(1, 3);
  INSERT INTO t1 VALUES(2, 4);
  INSERT INTO t1 VALUES(1, 5);
  INSERT INTO t1 VALUES(2, 6);
} {}

foreach {tn sql res} {
  1 "SELECT a, sum(b) FROM t1 GROUP BY a HAVING a=2" {2 12}
  2 "SELECT a, sum(b) FROM t1 GROUP BY a HAVING a=2 AND sum(b)>10" {2 12}
  3 "SELECT a, sum(b) FROM t1 GROUP BY a HAVING sum(b)>12" {}
} {
  do_execsql_test 1.$tn $sql $res
}

# Run an EXPLAIN command for both SQL statements. Return true if 
# the outputs are identical, or false otherwise.
#
proc compare_vdbe {sql1 sql2} {
  set r1 [list]
  set r2 [list]
  db eval "explain $sql1" { lappend r1 $opcode $p1 $p2 $p3 $p4 $p5}
  db eval "explain $sql2" { lappend r2 $opcode $p1 $p2 $p3 $p4 $p5}
  return [expr {$r1==$r2}]
}

proc do_compare_vdbe_test {tn sql1 sql2 res} {
  uplevel [list do_test $tn [list compare_vdbe $sql1 $sql2] $res]
}

#-------------------------------------------------------------------------
# Test that various statements that are eligible for the optimization
# produce the same VDBE code as optimizing by hand does.
#
foreach {tn sql1 sql2} {
  1 "SELECT a, sum(b) FROM t1 GROUP BY a HAVING a=2"
    "SELECT a, sum(b) FROM t1 WHERE a=2 GROUP BY a"

  2 "SELECT a, sum(b) FROM t1 GROUP BY a HAVING sum(b)>5 AND a=2"
    "SELECT a, sum(b) FROM t1 WHERE a=2 GROUP BY a HAVING sum(b)>5"

  3 "SELECT a, sum(b) FROM t1 GROUP BY a COLLATE binary HAVING a=2"
    "SELECT a, sum(b) FROM t1 WHERE a=2 GROUP BY a COLLATE binary"

  4 {
      SELECT x,y FROM (
        SELECT a AS x, sum(b) AS y FROM t1 
        GROUP BY a
      ) WHERE x BETWEEN 8888 AND 9999
    } {
      SELECT x,y FROM (
        SELECT a AS x, sum(b) AS y FROM t1 
        WHERE x BETWEEN 8888 AND 9999 
        GROUP BY a
      )
    }

  5 "SELECT a, sum(b) FROM t1 GROUP BY a COLLATE binary HAVING 0"
    "SELECT a, sum(b) FROM t1 WHERE 0 GROUP BY a COLLATE binary"

  6 "SELECT count(*) FROM t1,t2 WHERE a=c GROUP BY b, d HAVING b=d"
    "SELECT count(*) FROM t1,t2 WHERE a=c AND b=d GROUP BY b, d"

  7 {
      SELECT count(*) FROM t1,t2 WHERE a=c GROUP BY b, d 
      HAVING b=d COLLATE nocase
    } {
      SELECT count(*) FROM t1,t2 WHERE a=c AND b=d COLLATE nocase 
      GROUP BY b, d
    }

  8 "SELECT a, sum(b) FROM t1 GROUP BY a||b HAVING substr(a||b, 1, 1)='a'"
    "SELECT a, sum(b) FROM t1 WHERE substr(a||b, 1, 1)='a' GROUP BY a||b"
} {
  do_compare_vdbe_test 2.$tn $sql1 $sql2 1
}

#-------------------------------------------------------------------------
# 1: Test that the optimization is only applied if the GROUP BY term
#    uses BINARY collation.
#
# 2: Not applied if there is a non-deterministic function in the HAVING
#    term.
#
foreach {tn sql1 sql2} {
  1 "SELECT a, sum(b) FROM t1 GROUP BY a COLLATE nocase HAVING a=2"
    "SELECT a, sum(b) FROM t1 WHERE a=2 GROUP BY a COLLATE nocase"

  2 "SELECT a, sum(b) FROM t1 GROUP BY a HAVING randomblob(a)<X'88'"
    "SELECT a, sum(b) FROM t1 WHERE randomblob(a)<X'88' GROUP BY a"
} {
  do_compare_vdbe_test 3.$tn $sql1 $sql2 0
}


#-------------------------------------------------------------------------
# Test that non-deterministic functions disqualify a term from being
# moved from the HAVING to WHERE clause.
#
do_execsql_test 4.1 {
  CREATE TABLE t3(a, b);
  INSERT INTO t3 VALUES(1, 1);
  INSERT INTO t3 VALUES(1, 2);
  INSERT INTO t3 VALUES(1, 3);
  INSERT INTO t3 VALUES(2, 1);
  INSERT INTO t3 VALUES(2, 2);
  INSERT INTO t3 VALUES(2, 3);
}

proc nondeter {args} {
  incr ::nondeter_ret
  expr {$::nondeter_ret % 2}
}
db func nondeter nondeter

set ::nondeter_ret 0
do_execsql_test 4.2 {
  SELECT a, sum(b) FROM t3 GROUP BY a HAVING nondeter(a)
} {1 6}

# If the term where moved, the query above would return the same
# result as the following. But it does not.
#
set ::nondeter_ret 0
do_execsql_test 4.3 {
  SELECT a, sum(b) FROM t3 WHERE nondeter(a) GROUP BY a
} {1 4 2 2}


finish_test