SQLite

    if( (exprTableUsage(pMaskSet, pList->a[iFirst++].pExpr)&allowed)!=0 ){
      return 1;
    }
  }
  return 0;
}

/*
** This function searches the expression list passed as the second argument
** for an expression of type TK_COLUMN that refers to the same column and
** uses the same collation sequence as the iCol'th column of index pIdx.
** Argument iBase is the cursor number used for the table that pIdx refers
** to.
**
** If such an expression is found, its index in pList->a[] is returned. If
** no expression is found, -1 is returned.
*/
static int findIndexCol(
  Parse *pParse,                  /* Parse context */
  ExprList *pList,                /* Expression list to search */
  int iBase,                      /* Cursor for table associated with pIdx */
  Index *pIdx,                    /* Index to match column of */
  int iCol                        /* Column of index to match */
){
  int i;
  const char *zColl = pIdx->azColl[iCol];

  for(i=0; i<pList->nExpr; i++){
    Expr *p = pList->a[i].pExpr;
    if( pIdx->aiColumn[iCol]==p->iColumn && iBase==p->iTable ){
      CollSeq *pColl = sqlite3ExprCollSeq(pParse, p);
      if( pColl && 0==sqlite3StrICmp(pColl->zName, zColl) ){
        return i;
      }
    }
  }

  return -1;
}

/*
** This routine determines if pIdx can be used to assist in processing a
** DISTINCT qualifier. In other words, it tests whether or not using this
** index for the outer loop guarantees that rows with equal values for
** all expressions in the pDistinct list are delivered grouped together.
**
** For example, the query 
**
**   SELECT DISTINCT a, b, c FROM tbl WHERE a = ?
**
** can benefit from any index on columns "b" and "c".
*/
static int isDistinctIndex(
  Parse *pParse,                  /* Parsing context */
  WhereClause *pWC,               /* The WHERE clause */
  Index *pIdx,                    /* The index being considered */
  int base,                       /* Cursor number for the table pIdx is on */
  ExprList *pDistinct,            /* The DISTINCT expressions */
  int nEqCol                      /* Number of index columns with == */
){
  Bitmask mask = 0;               /* Mask of unaccounted for pDistinct exprs */
  int i;                          /* Iterator variable */

  if( pIdx->zName==0 || pDistinct==0 || pDistinct->nExpr>=BMS ) return 0;

  /* Loop through all the expressions in the distinct list. If any of them
  ** are not simple column references, return early. Otherwise, test if the
  ** WHERE clause contains a "col=X" clause. If it does, the expression
  ** can be ignored. If it does not, and the column does not belong to the
  ** same table as index pIdx, return early. Finally, if there is no
  ** matching "col=X" expression and the column is on the same table as pIdx,
  ** set the corresponding bit in variable mask.
  */
  for(i=0; i<pDistinct->nExpr; i++){
    WhereTerm *pTerm;
    Expr *p = pDistinct->a[i].pExpr;
    if( p->op!=TK_COLUMN ) return 0;
    pTerm = findTerm(pWC, p->iTable, p->iColumn, ~(Bitmask)0, WO_EQ, 0);
    if( pTerm ){
      Expr *pX = pTerm->pExpr;
      CollSeq *p1 = sqlite3BinaryCompareCollSeq(pParse, pX->pLeft, pX->pRight);
      CollSeq *p2 = sqlite3ExprCollSeq(pParse, p);
      if( p1==p2 ) continue;
    }
    if( p->iTable!=base ) return 0;
    mask |= (((Bitmask)1) << i);
  }

  for(i=nEqCol; mask && i<pIdx->nColumn; i++){
    int iExpr = findIndexCol(pParse, pDistinct, base, pIdx, i);
    if( iExpr<0 ) break;
    mask &= ~(((Bitmask)1) << iExpr);
  }

  return (mask==0);
}


/*
** Return true if the DISTINCT expression-list passed as the third argument
** is redundant. A DISTINCT list is redundant if the database contains a
** UNIQUE index that guarantees that the result of the query will be distinct
** anyway.
*/
static int isDistinctRedundant(
  Parse *pParse,
  SrcList *pTabList,
  WhereClause *pWC,
  ExprList *pDistinct
){
  Table *pTab;
  Index *pIdx;
  int i;                          
  int iBase;

  /* If there is more than one table or sub-select in the FROM clause of
  ** this query, then it will not be possible to show that the DISTINCT 
  ** clause is redundant. */
  if( pTabList->nSrc!=1 ) return 0;
  iBase = pTabList->a[0].iCursor;
  pTab = pTabList->a[0].pTab;

  /* If any of the expressions is an IPK column, then return true. */
  for(i=0; i<pDistinct->nExpr; i++){
    Expr *p = pDistinct->a[i].pExpr;
    assert( p->op!=TK_COLUMN || p->iTable==iBase );
    if( p->op==TK_COLUMN && p->iColumn<0 ) return 1;
  }

  /* Loop through all indices on the table, checking each to see if it makes
  ** the DISTINCT qualifier redundant. It does so if:
  **
  **   1. The index is itself UNIQUE, and
  **
  **   2. All of the columns in the index are either part of the pDistinct
  **      list, or else the WHERE clause contains a term of the form "col=X",
  **      where X is a constant value. The collation sequences of the
  **      comparison and select-list expressions must match those of the index.
  */
  for(pIdx=pTab->pIndex; pIdx; pIdx=pIdx->pNext){
    if( pIdx->onError==OE_None ) continue;
    for(i=0; i<pIdx->nColumn; i++){
      int iCol = pIdx->aiColumn[i];
      if( 0==findTerm(pWC, iBase, iCol, ~(Bitmask)0, WO_EQ, pIdx) 
       && 0>findIndexCol(pParse, pDistinct, iBase, pIdx, i)
      ){
        break;
      }
    }
    if( i==pIdx->nColumn ){
      /* This index implies that the DISTINCT qualifier is redundant. */
      return 1;
    }
  }

  return 0;
}

/*
** This routine decides if pIdx can be used to satisfy the ORDER BY
** clause.  If it can, it returns 1.  If pIdx cannot satisfy the
** ORDER BY clause, this routine returns 0.
**
** pOrderBy is an ORDER BY clause from a SELECT statement.  pTab is the

      wsFlags |= WHERE_ROWID_RANGE|WHERE_COLUMN_RANGE|WHERE_ORDERBY;
      wsFlags |= (rev ? WHERE_REVERSE : 0);
    }

    /* If there is a DISTINCT qualifier and this index will scan rows in
    ** order of the DISTINCT expressions, clear bDist and set the appropriate
    ** flags in wsFlags. */

    if( isDistinctIndex(pParse, pWC, pProbe, iCur, pDistinct, nEq) ){

      bDist = 0;
      wsFlags |= WHERE_ROWID_RANGE|WHERE_COLUMN_RANGE|WHERE_DISTINCT;
    }

    /* If currently calculating the cost of using an index (not the IPK
    ** index), determine if all required column data may be obtained without 
    ** using the main table (i.e. if the index is a covering

      }
    }
    whereClauseClear(pWInfo->pWC);
    sqlite3DbFree(db, pWInfo);
  }
}







































































/*
** Generate the beginning of the loop used for WHERE clause processing.
** The return value is a pointer to an opaque structure that contains
** information needed to terminate the loop.  Later, the calling routine
** should invoke sqlite3WhereEnd() with the return value of this function
** in order to complete the WHERE clause processing.

    goto whereBeginError;
  }

  /* Check if the DISTINCT qualifier, if there is one, is redundant. 
  ** If it is, then set pDistinct to NULL and WhereInfo.eDistinct to
  ** WHERE_DISTINCT_UNIQUE to tell the caller to ignore the DISTINCT.
  */
  if( pDistinct && isDistinctRedundant(pParse, pTabList, pWC, pDistinct) ){
    pDistinct = 0;
    pWInfo->eDistinct = WHERE_DISTINCT_UNIQUE;
  }

  /* Chose the best index to use for each table in the FROM clause.
  **
  ** This loop fills in the following fields:

proc do_distinct_noop_test {tn sql} {
  uplevel [list do_test $tn [list is_distinct_noop $sql] 1]
}
proc do_distinct_not_noop_test {tn sql} {
  uplevel [list do_test $tn [list is_distinct_noop $sql] 0]
}

proc do_temptables_test {tn sql temptables} {
  uplevel [list do_test $tn [subst -novar {
    set ret ""
    db eval "EXPLAIN [set sql]" {
      if {$opcode == "OpenEphemeral"} { 
        if {$p5 != "10" && $p5!="00"} { error "p5 = $p5" }
        if {$p5 == "10"} {
          lappend ret hash
        } else {
          lappend ret btree
        }
      }
    }
    set ret
  }] $temptables]
}


#-------------------------------------------------------------------------
# The following tests - distinct-1.* - check that the planner correctly 
# detects cases where a UNIQUE index means that a DISTINCT clause is 
# redundant. Currently the planner only detects such cases when there
# is a single table in the FROM clause.
#

  if {$noop} {
    do_distinct_noop_test 1.$tn $sql
  } else {
    do_distinct_not_noop_test 1.$tn $sql
  }
}

#-------------------------------------------------------------------------
# The following tests - distinct-2.* - test cases where an index is
# used to deliver results in order of the DISTINCT expressions. 
#
drop_all_tables
do_execsql_test 2.0 {
  CREATE TABLE t1(a, b, c);

  CREATE INDEX i1 ON t1(a, b);
  CREATE INDEX i2 ON t1(b COLLATE nocase, c COLLATE nocase);

  INSERT INTO t1 VALUES('a', 'b', 'c');
  INSERT INTO t1 VALUES('A', 'B', 'C');
  INSERT INTO t1 VALUES('a', 'b', 'c');
  INSERT INTO t1 VALUES('A', 'B', 'C');
}

foreach {tn sql temptables res} {
  1   "a, b FROM t1"                                       {}      {A B a b}
  2   "b, a FROM t1"                                       {}      {B A b a}
  3   "a, b, c FROM t1"                                    {hash}  {a b c A B C}
  4   "a, b, c FROM t1 ORDER BY a, b, c"                   {btree} {A B C a b c}
  5   "b FROM t1 WHERE a = 'a'"                            {}      {b}
  6   "b FROM t1"                                          {hash}  {b B}
  7   "a FROM t1"                                          {}      {A a}
  8   "b COLLATE nocase FROM t1"                           {}      {b}
  9   "b COLLATE nocase FROM t1 ORDER BY b COLLATE nocase" {}      {B}
} {
  do_execsql_test    2.$tn.1 "SELECT DISTINCT $sql" $res
  do_temptables_test 2.$tn.2 "SELECT DISTINCT $sql" $temptables
}





finish_test

#-------------------------------------------------------------------------
# The commands provided by the code in this file to help with creating 
# test cases are as follows:
#
# Commands to manipulate the db and the file-system at a high level:
#
#      copy_file              FROM TO
#      drop_all_tables        ?DB?
#      forcedelete            FILENAME
#
# Test the capability of the SQLite version built into the interpreter to
# determine if a specific test can be run:
#
#      ifcapable              EXPR
#