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Comment:Avoid redundant ORDER BY operations when rewriting SELECT statements that contain window functions.
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SHA3-256:336de43a47e206fe7629072e5b8c80d4ede17ead8ef4dcf5d8da6833ff22d2f9
User & Date: dan 2018-06-27 19:48:50
Context
2018-06-27
20:24
Add missing VdbeCoverage() and VdbeCoverageNeverTaken() macros to window.c. check-in: 4383cb68 user: dan tags: exp-window-functions
19:48
Avoid redundant ORDER BY operations when rewriting SELECT statements that contain window functions. check-in: 336de43a user: dan tags: exp-window-functions
2018-06-26
20:19
Merge latest trunk changes. check-in: d9f814b4 user: dan tags: exp-window-functions
Changes
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Changes to src/select.c.

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    /* If ORDER BY makes no difference in the output then neither does
    ** DISTINCT so it can be removed too. */
    sqlite3ExprListDelete(db, p->pOrderBy);
    p->pOrderBy = 0;
    p->selFlags &= ~SF_Distinct;
  }
  sqlite3SelectPrep(pParse, p, 0);
  memset(&sSort, 0, sizeof(sSort));
  sSort.pOrderBy = p->pOrderBy;
  if( pParse->nErr || db->mallocFailed ){
    goto select_end;
  }
  assert( p->pEList!=0 );
#if SELECTTRACE_ENABLED
  if( sqlite3SelectTrace & 0x104 ){
    SELECTTRACE(0x104,pParse,p, ("after name resolution:\n"));
................................................................................
    SELECTTRACE(0x104,pParse,p, ("after window rewrite:\n"));
    sqlite3TreeViewSelect(0, p, 0);
  }
#endif
#endif /* SQLITE_OMIT_WINDOWFUNC */
  pTabList = p->pSrc;
  isAgg = (p->selFlags & SF_Aggregate)!=0;



  /* Try to various optimizations (flattening subqueries, and strength
  ** reduction of join operators) in the FROM clause up into the main query
  */
#if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW)
  for(i=0; !p->pPrior && i<pTabList->nSrc; i++){
    struct SrcList_item *pItem = &pTabList->a[i];







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    /* If ORDER BY makes no difference in the output then neither does
    ** DISTINCT so it can be removed too. */
    sqlite3ExprListDelete(db, p->pOrderBy);
    p->pOrderBy = 0;
    p->selFlags &= ~SF_Distinct;
  }
  sqlite3SelectPrep(pParse, p, 0);


  if( pParse->nErr || db->mallocFailed ){
    goto select_end;
  }
  assert( p->pEList!=0 );
#if SELECTTRACE_ENABLED
  if( sqlite3SelectTrace & 0x104 ){
    SELECTTRACE(0x104,pParse,p, ("after name resolution:\n"));
................................................................................
    SELECTTRACE(0x104,pParse,p, ("after window rewrite:\n"));
    sqlite3TreeViewSelect(0, p, 0);
  }
#endif
#endif /* SQLITE_OMIT_WINDOWFUNC */
  pTabList = p->pSrc;
  isAgg = (p->selFlags & SF_Aggregate)!=0;
  memset(&sSort, 0, sizeof(sSort));
  sSort.pOrderBy = p->pOrderBy;

  /* Try to various optimizations (flattening subqueries, and strength
  ** reduction of join operators) in the FROM clause up into the main query
  */
#if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW)
  for(i=0; !p->pPrior && i<pTabList->nSrc; i++){
    struct SrcList_item *pItem = &pTabList->a[i];

Changes to src/window.c.

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    Window *pMWin = p->pWin;      /* Master window object */
    Window *pWin;                 /* Window object iterator */

    p->pSrc = 0;
    p->pWhere = 0;
    p->pGroupBy = 0;
    p->pHaving = 0;













    /* Assign a cursor number for the ephemeral table used to buffer rows.
    ** The OpenEphemeral instruction is coded later, after it is known how
    ** many columns the table will have.  */
    pMWin->iEphCsr = pParse->nTab++;

    selectWindowRewriteEList(pParse, pMWin, p->pEList, &pSublist);
    selectWindowRewriteEList(pParse, pMWin, p->pOrderBy, &pSublist);
    pMWin->nBufferCol = (pSublist ? pSublist->nExpr : 0);

    /* Create the ORDER BY clause for the sub-select. This is the concatenation
    ** of the window PARTITION and ORDER BY clauses. Append the same 
    ** expressions to the sub-select expression list. They are required to
    ** figure out where boundaries for partitions and sets of peer rows.  */
    pSort = sqlite3ExprListDup(db, pMWin->pPartition, 0);
    if( pMWin->pOrderBy ){
      pSort = exprListAppendList(pParse, pSort, pMWin->pOrderBy);
    }
    pSublist = exprListAppendList(pParse, pSublist, pSort);

    /* Append the arguments passed to each window function to the
    ** sub-select expression list. Also allocate two registers for each
    ** window function - one for the accumulator, another for interim
    ** results.  */
    for(pWin=pMWin; pWin; pWin=pWin->pNextWin){
      pWin->iArgCol = (pSublist ? pSublist->nExpr : 0);







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    Window *pMWin = p->pWin;      /* Master window object */
    Window *pWin;                 /* Window object iterator */

    p->pSrc = 0;
    p->pWhere = 0;
    p->pGroupBy = 0;
    p->pHaving = 0;

    /* Create the ORDER BY clause for the sub-select. This is the concatenation
    ** of the window PARTITION and ORDER BY clauses. Then, if this makes it
    ** redundant, remove the ORDER BY from the parent SELECT.  */
    pSort = sqlite3ExprListDup(db, pMWin->pPartition, 0);
    pSort = exprListAppendList(pParse, pSort, pMWin->pOrderBy);
    if( pSort && p->pOrderBy ){
      if( sqlite3ExprListCompare(pSort, p->pOrderBy, -1)==0 ){
        sqlite3ExprListDelete(db, p->pOrderBy);
        p->pOrderBy = 0;
      }
    }

    /* Assign a cursor number for the ephemeral table used to buffer rows.
    ** The OpenEphemeral instruction is coded later, after it is known how
    ** many columns the table will have.  */
    pMWin->iEphCsr = pParse->nTab++;

    selectWindowRewriteEList(pParse, pMWin, p->pEList, &pSublist);
    selectWindowRewriteEList(pParse, pMWin, p->pOrderBy, &pSublist);
    pMWin->nBufferCol = (pSublist ? pSublist->nExpr : 0);

    /* Append the PARTITION BY and ORDER BY expressions to the to the 

    ** sub-select expression list. They are required to figure out where 
    ** boundaries for partitions and sets of peer rows lie.  */


    pSublist = exprListAppendList(pParse, pSublist, pMWin->pPartition);

    pSublist = exprListAppendList(pParse, pSublist, pMWin->pOrderBy);

    /* Append the arguments passed to each window function to the
    ** sub-select expression list. Also allocate two registers for each
    ** window function - one for the accumulator, another for interim
    ** results.  */
    for(pWin=pMWin; pWin; pWin=pWin->pNextWin){
      pWin->iArgCol = (pSublist ? pSublist->nExpr : 0);