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Overview
Comment:Use a hash table to improve the preformance of column name uniqueness checking.
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SHA1:5b08f29f458c600401860c7d70d8174cf61e69f8
User & Date: drh 2015-11-14 20:52:43
Context
2015-11-14
22:04
Code simplification in sqlite3ColumnsFromExprList(). Update the %z format code so that it works with buffers obtained from sqlite3DbMalloc(). Add a testcase for the slow column name uniquifier. check-in: 92724260 user: drh tags: trunk
20:52
Use a hash table to improve the preformance of column name uniqueness checking. check-in: 5b08f29f user: drh tags: trunk
16:47
Use randomness to prevent slowness in the generated-column-name uniqueness checking. check-in: 62667129 user: drh tags: trunk
Changes
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Changes to src/select.c.

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  int i, j;                   /* Loop counters */
  u32 cnt;                    /* Index added to make the name unique */
  Column *aCol, *pCol;        /* For looping over result columns */
  int nCol;                   /* Number of columns in the result set */
  Expr *p;                    /* Expression for a single result column */
  char *zName;                /* Column name */
  int nName;                  /* Size of name in zName[] */



  if( pEList ){
    nCol = pEList->nExpr;
    aCol = sqlite3DbMallocZero(db, sizeof(aCol[0])*nCol);
    testcase( aCol==0 );
  }else{
    nCol = 0;
    aCol = 0;
  }
  *pnCol = nCol;
  *paCol = aCol;

  for(i=0, pCol=aCol; i<nCol; i++, pCol++){
    /* Get an appropriate name for the column
    */
    p = sqlite3ExprSkipCollate(pEList->a[i].pExpr);
    if( (zName = pEList->a[i].zName)!=0 ){
      /* If the column contains an "AS <name>" phrase, use <name> as the name */
      zName = sqlite3DbStrDup(db, zName);
    }else{
................................................................................
        assert( !ExprHasProperty(pColExpr, EP_IntValue) );
        zName = sqlite3MPrintf(db, "%s", pColExpr->u.zToken);
      }else{
        /* Use the original text of the column expression as its name */
        zName = sqlite3MPrintf(db, "%s", pEList->a[i].zSpan);
      }
    }
    if( db->mallocFailed ){
      sqlite3DbFree(db, zName);
      break;
    }

    /* Make sure the column name is unique.  If the name is not unique,
    ** append an integer to the name so that it becomes unique.
    */
    nName = sqlite3Strlen30(zName);
    for(j=cnt=0; j<i; j++){
      if( sqlite3StrICmp(aCol[j].zName, zName)==0 ){
        char *zNewName;
        int k;

        for(k=nName-1; k>1 && sqlite3Isdigit(zName[k]); k--){}
        if( k>=0 && zName[k]==':' ) nName = k;
        zName[nName] = 0;
        zNewName = sqlite3MPrintf(db, "%s:%u", zName, ++cnt);
        sqlite3DbFree(db, zName);
        zName = zNewName;
        j = -1;
        if( zName==0 ) break;
        if( cnt>3 ) sqlite3_randomness(sizeof(cnt), &cnt);
      }
    }
    pCol->zName = zName;


  }


  if( db->mallocFailed ){
    for(j=0; j<i; j++){
      sqlite3DbFree(db, aCol[j].zName);
    }
    sqlite3DbFree(db, aCol);
    *paCol = 0;
    *pnCol = 0;







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  int i, j;                   /* Loop counters */
  u32 cnt;                    /* Index added to make the name unique */
  Column *aCol, *pCol;        /* For looping over result columns */
  int nCol;                   /* Number of columns in the result set */
  Expr *p;                    /* Expression for a single result column */
  char *zName;                /* Column name */
  int nName;                  /* Size of name in zName[] */
  Hash ht;                    /* Hash table of column names */

  sqlite3HashInit(&ht);
  if( pEList ){
    nCol = pEList->nExpr;
    aCol = sqlite3DbMallocZero(db, sizeof(aCol[0])*nCol);
    testcase( aCol==0 );
  }else{
    nCol = 0;
    aCol = 0;
  }
  *pnCol = nCol;
  *paCol = aCol;

  for(i=0, pCol=aCol; i<nCol && !db->mallocFailed; i++, pCol++){
    /* Get an appropriate name for the column
    */
    p = sqlite3ExprSkipCollate(pEList->a[i].pExpr);
    if( (zName = pEList->a[i].zName)!=0 ){
      /* If the column contains an "AS <name>" phrase, use <name> as the name */
      zName = sqlite3DbStrDup(db, zName);
    }else{
................................................................................
        assert( !ExprHasProperty(pColExpr, EP_IntValue) );
        zName = sqlite3MPrintf(db, "%s", pColExpr->u.zToken);
      }else{
        /* Use the original text of the column expression as its name */
        zName = sqlite3MPrintf(db, "%s", pEList->a[i].zSpan);
      }
    }





    /* Make sure the column name is unique.  If the name is not unique,
    ** append an integer to the name so that it becomes unique.
    */
    cnt = 0;
    while( zName && sqlite3HashFind(&ht, zName)!=0 ){

      char *zNewName;

      nName = sqlite3Strlen30(zName);
      for(j=nName-1; j>0 && sqlite3Isdigit(zName[j]); j--){}
      if( zName[j]==':' ) nName = j;
      zName[nName] = 0;
      zNewName = sqlite3MPrintf(db, "%s:%u", zName, ++cnt);
      sqlite3DbFree(db, zName);
      zName = zNewName;


      if( cnt>3 ) sqlite3_randomness(sizeof(cnt), &cnt);
    }

    pCol->zName = zName;
    if( zName && sqlite3HashInsert(&ht, zName, pCol)==pCol ){
      db->mallocFailed = 1;
    }
  }
  sqlite3HashClear(&ht);
  if( db->mallocFailed ){
    for(j=0; j<i; j++){
      sqlite3DbFree(db, aCol[j].zName);
    }
    sqlite3DbFree(db, aCol);
    *paCol = 0;
    *pnCol = 0;