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Overview
Comment:Add the count-of-view optimization when compiled using SQLITE_COUNTOFVIEW_OPTIMIZATION.
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Timelines: family | ancestors | descendants | both | branch-3.8.9
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SHA3-256: b7ae4b879fc086e9543493843377ae90ceff1fe49c97b4c23367012034c3c9d5
User & Date: drh 2017-07-05 14:54:24
Context
2017-07-05
16:20
Make use of covering indexes in the OR optimization. check-in: fcbd6abd user: drh tags: branch-3.8.9
14:54
Add the count-of-view optimization when compiled using SQLITE_COUNTOFVIEW_OPTIMIZATION. check-in: b7ae4b87 user: drh tags: branch-3.8.9
2017-07-04
19:34
Add the count-of-view optimization when compiled using SQLITE_COUNTOFVIEW_OPTIMIZATION. check-in: d1ba2002 user: drh tags: trunk
2017-06-06
18:22
Add the SQLITE_DEFAULT_ROWEST compile-time option for changing the estimated number of rows in tables that lack sqlite_stat1 entries. check-in: 802b82f3 user: drh tags: branch-3.8.9
Changes
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Changes to src/select.c.

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    );
  }
}
#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.
**
** This routine returns the number of errors.  If any errors are
................................................................................
#if SELECTTRACE_ENABLED
    SELECTTRACE(1,pParse,p,("end compound-select processing\n"));
    pParse->nSelectIndent--;
#endif
    return rc;
  }
#endif











  /* If the query is DISTINCT with an ORDER BY but is not an aggregate, and 
  ** if the select-list is the same as the ORDER BY list, then this query
  ** can be rewritten as a GROUP BY. In other words, this:
  **
  **     SELECT DISTINCT xyz FROM ... ORDER BY xyz
  **







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    );
  }
}
#else
# define explainSimpleCount(a,b,c)
#endif

#ifdef SQLITE_COUNTOFVIEW_OPTIMIZATION
/*
** Attempt to transform a query of the form
**
**    SELECT count(*) FROM (SELECT x FROM t1 UNION ALL SELECT y FROM t2)
**
** Into this:
**
**    SELECT (SELECT count(*) FROM t1)+(SELECT count(*) FROM t2)
**
** The transformation only works if all of the following are true:
**
**   *  The subquery is a UNION ALL of two or more terms
**   *  There is no WHERE or GROUP BY or HAVING clauses on the subqueries
**   *  The outer query is a simple count(*)
**
** Return TRUE if the optimization is undertaken.
*/
static int countOfViewOptimization(Parse *pParse, Select *p){
  Select *pSub, *pPrior;
  Expr *pExpr;
  Expr *pCount;
  sqlite3 *db;
  if( (p->selFlags & SF_Aggregate)==0 ) return 0;   /* This is an aggregate query */
  if( p->pEList->nExpr!=1 ) return 0;               /* Single result column */
  pExpr = p->pEList->a[0].pExpr;
  if( pExpr->op!=TK_AGG_FUNCTION ) return 0;        /* Result is an aggregate */
  if( sqlite3_stricmp(pExpr->u.zToken,"count") ) return 0;  /* Must be count() */
  if( pExpr->x.pList!=0 ) return 0;                 /* Must be count(*) */
  if( p->pSrc->nSrc!=1 ) return 0;                  /* One table in the FROM clause */
  pSub = p->pSrc->a[0].pSelect;
  if( pSub==0 ) return 0;                           /* The FROM is a subquery */
  if( pSub->pPrior==0 ) return 0;                   /* Must be a compound subquery */
  do{
    if( pSub->op!=TK_ALL && pSub->pPrior ) return 0;  /* Must be UNION ALL */
    if( pSub->pWhere ) return 0;                      /* No WHERE clause */
    if( pSub->selFlags & SF_Aggregate ) return 0;     /* Not an aggregate */
    pSub = pSub->pPrior;                              /* Repeat over compound terms */
  }while( pSub );

  /* If we reach this point, that means it is OK to perform the transformation */

  db = pParse->db;
  pCount = pExpr;
  pExpr = 0;
  pSub = p->pSrc->a[0].pSelect;
  p->pSrc->a[0].pSelect = 0;
  sqlite3SrcListDelete(db, p->pSrc);
  p->pSrc = sqlite3DbMallocZero(pParse->db, sizeof(*p->pSrc));
  while( pSub ){
    Expr *pTerm;
    pPrior = pSub->pPrior;
    pSub->pPrior = 0;
    pSub->pNext = 0;
    pSub->selFlags |= SF_Aggregate;
    pSub->selFlags &= ~SF_Compound;
    pSub->nSelectRow = 0;
    sqlite3ExprListDelete(db, pSub->pEList);
    pTerm = pPrior ? sqlite3ExprDup(db, pCount, 0) : pCount;
    pSub->pEList = sqlite3ExprListAppend(pParse, 0, pTerm);
    pTerm = sqlite3PExpr(pParse, TK_SELECT, 0, 0, 0);
    if( pTerm ){
      pTerm->x.pSelect = pSub;
      ExprSetProperty(pTerm, EP_xIsSelect|EP_Subquery);
      sqlite3ExprSetHeightAndFlags(pParse, pTerm);
      if( pExpr==0 ){
        pExpr = pTerm;
      }else{
        pExpr = sqlite3PExpr(pParse, TK_PLUS, pTerm, pExpr, 0);
      }
    }else{
      sqlite3SelectDelete(db, pSub);
    }
    pSub = pPrior;
  }
  p->pEList->a[0].pExpr = pExpr;
  p->selFlags &= ~SF_Aggregate;

#if SELECTTRACE_ENABLED
  if( sqlite3SelectTrace & 0x400 ){
    SELECTTRACE(0x400,pParse,p,("After count-of-view optimization:\n"));
    sqlite3TreeViewSelect(0, p, 0);
  }
#endif
  return 1;
}
#endif /* SQLITE_COUNTOFVIEW_OPTIMIZATION */

/*
** 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.
**
** This routine returns the number of errors.  If any errors are
................................................................................
#if SELECTTRACE_ENABLED
    SELECTTRACE(1,pParse,p,("end compound-select processing\n"));
    pParse->nSelectIndent--;
#endif
    return rc;
  }
#endif

#ifdef SQLITE_COUNTOFVIEW_OPTIMIZATION
  if( OptimizationEnabled(db, SQLITE_QueryFlattener|SQLITE_CountOfView)
   && countOfViewOptimization(pParse, p)
  ){
    if( db->mallocFailed ) goto select_end;
    pEList = p->pEList;
    pTabList = p->pSrc;
  }
#endif

  /* If the query is DISTINCT with an ORDER BY but is not an aggregate, and 
  ** if the select-list is the same as the ORDER BY list, then this query
  ** can be rewritten as a GROUP BY. In other words, this:
  **
  **     SELECT DISTINCT xyz FROM ... ORDER BY xyz
  **

Changes to src/sqliteInt.h.

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#define SQLITE_DistinctOpt    0x0020   /* DISTINCT using indexes */
#define SQLITE_CoverIdxScan   0x0040   /* Covering index scans */
#define SQLITE_OrderByIdxJoin 0x0080   /* ORDER BY of joins via index */
#define SQLITE_SubqCoroutine  0x0100   /* Evaluate subqueries as coroutines */
#define SQLITE_Transitive     0x0200   /* Transitive constraints */
#define SQLITE_OmitNoopJoin   0x0400   /* Omit unused tables in joins */
#define SQLITE_Stat34         0x0800   /* Use STAT3 or STAT4 data */

#define SQLITE_AllOpts        0xffff   /* All optimizations */

/*
** Macros for testing whether or not optimizations are enabled or disabled.
*/
#ifndef SQLITE_OMIT_BUILTIN_TEST
#define OptimizationDisabled(db, mask)  (((db)->dbOptFlags&(mask))!=0)







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#define SQLITE_DistinctOpt    0x0020   /* DISTINCT using indexes */
#define SQLITE_CoverIdxScan   0x0040   /* Covering index scans */
#define SQLITE_OrderByIdxJoin 0x0080   /* ORDER BY of joins via index */
#define SQLITE_SubqCoroutine  0x0100   /* Evaluate subqueries as coroutines */
#define SQLITE_Transitive     0x0200   /* Transitive constraints */
#define SQLITE_OmitNoopJoin   0x0400   /* Omit unused tables in joins */
#define SQLITE_Stat34         0x0800   /* Use STAT3 or STAT4 data */
#define SQLITE_CountOfView    0x1000   /* The count-of-view optimization */
#define SQLITE_AllOpts        0xffff   /* All optimizations */

/*
** Macros for testing whether or not optimizations are enabled or disabled.
*/
#ifndef SQLITE_OMIT_BUILTIN_TEST
#define OptimizationDisabled(db, mask)  (((db)->dbOptFlags&(mask))!=0)