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Comment:Make sure that a DISTINCT query with an ORDER BY works correctly even if it uses a descending index. Fix for ticket [c5ea805691bfc4204b1cb9e].
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SHA1:0d3aef97ebddf422b8bdcbc5878970c6129e3f54
User & Date: drh 2014-12-04 21:54:58
Context
2014-12-05
00:17
Fix the autoconf and MSVC makefiles, which have been broken for nearly a month. :-( check-in: 520c2b83 user: drh tags: trunk
2014-12-04
23:42
Import from trunk support for SQLITE_CHECKPOINT_TRUNCATE and fixes for a couple of obscure bugs. check-in: 463ad971 user: drh tags: apple-osx
23:35
Incorporate the SQLITE_CHECKPOINT_TRUNCATE enhancement and a couple of obscure bug fixes from trunk. check-in: 34ffa3b3 user: drh tags: sessions
21:54
Make sure that a DISTINCT query with an ORDER BY works correctly even if it uses a descending index. Fix for ticket [c5ea805691bfc4204b1cb9e]. check-in: 0d3aef97 user: drh tags: trunk
20:24
Performance enhancement for single-table queries with many OR-connected WHERE clause terms and multiple indexes with the same left-most columns. check-in: 1461d543 user: drh tags: trunk
Changes
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Changes to src/select.c.

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  ** 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
  **
  ** is transformed to:
  **
  **     SELECT xyz FROM ... GROUP BY xyz
  **
  ** The second form is preferred as a single index (or temp-table) may be 
  ** used for both the ORDER BY and DISTINCT processing. As originally 
  ** written the query must use a temp-table for at least one of the ORDER 
  ** BY and DISTINCT, and an index or separate temp-table for the other.
  */
  if( (p->selFlags & (SF_Distinct|SF_Aggregate))==SF_Distinct 
   && sqlite3ExprListCompare(sSort.pOrderBy, p->pEList, -1)==0
  ){
    p->selFlags &= ~SF_Distinct;
    p->pGroupBy = sqlite3ExprListDup(db, p->pEList, 0);
    pGroupBy = p->pGroupBy;
    sSort.pOrderBy = 0;
    /* Notice that even thought SF_Distinct has been cleared from p->selFlags,
    ** the sDistinct.isTnct is still set.  Hence, isTnct represents the
    ** original setting of the SF_Distinct flag, not the current setting */
    assert( sDistinct.isTnct );
  }

  /* If there is an ORDER BY clause, then this sorting







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  ** 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
  **
  ** is transformed to:
  **
  **     SELECT xyz FROM ... GROUP BY xyz ORDER BY xyz
  **
  ** The second form is preferred as a single index (or temp-table) may be 
  ** used for both the ORDER BY and DISTINCT processing. As originally 
  ** written the query must use a temp-table for at least one of the ORDER 
  ** BY and DISTINCT, and an index or separate temp-table for the other.
  */
  if( (p->selFlags & (SF_Distinct|SF_Aggregate))==SF_Distinct 
   && sqlite3ExprListCompare(sSort.pOrderBy, p->pEList, -1)==0
  ){
    p->selFlags &= ~SF_Distinct;
    p->pGroupBy = sqlite3ExprListDup(db, p->pEList, 0);
    pGroupBy = p->pGroupBy;

    /* Notice that even thought SF_Distinct has been cleared from p->selFlags,
    ** the sDistinct.isTnct is still set.  Hence, isTnct represents the
    ** original setting of the SF_Distinct flag, not the current setting */
    assert( sDistinct.isTnct );
  }

  /* If there is an ORDER BY clause, then this sorting

Changes to test/distinct.test.

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    SELECT DISTINCT
      CASE a WHEN 1 THEN x'0000000000'
             WHEN 2 THEN zeroblob(5)
             ELSE 'xyzzy' END
      FROM t1;
  SELECT quote(x) FROM t2 ORDER BY 1;
} {'xyzzy' X'0000000000'}































finish_test








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    SELECT DISTINCT
      CASE a WHEN 1 THEN x'0000000000'
             WHEN 2 THEN zeroblob(5)
             ELSE 'xyzzy' END
      FROM t1;
  SELECT quote(x) FROM t2 ORDER BY 1;
} {'xyzzy' X'0000000000'}

#----------------------------------------------------------------------------
# Ticket [c5ea805691bfc4204b1cb9e9aa0103bd48bc7d34] (2014-12-04)
# Make sure that DISTINCT works together with ORDER BY and descending
# indexes.
#
do_execsql_test 5.1 {
  DROP TABLE IF EXISTS t1;
  CREATE TABLE t1(x);
  INSERT INTO t1(x) VALUES(3),(1),(5),(2),(6),(4),(5),(1),(3);
  CREATE INDEX t1x ON t1(x DESC);
  SELECT DISTINCT x FROM t1 ORDER BY x ASC;
} {1 2 3 4 5 6}
do_execsql_test 5.2 {
  SELECT DISTINCT x FROM t1 ORDER BY x DESC;
} {6 5 4 3 2 1}
do_execsql_test 5.3 {
  SELECT DISTINCT x FROM t1 ORDER BY x;
} {1 2 3 4 5 6}
do_execsql_test 5.4 {
  DROP INDEX t1x;
  CREATE INDEX t1x ON t1(x ASC);
  SELECT DISTINCT x FROM t1 ORDER BY x ASC;
} {1 2 3 4 5 6}
do_execsql_test 5.5 {
  SELECT DISTINCT x FROM t1 ORDER BY x DESC;
} {6 5 4 3 2 1}
do_execsql_test 5.6 {
  SELECT DISTINCT x FROM t1 ORDER BY x;
} {1 2 3 4 5 6}

finish_test