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Overview
Comment:Make sure the ORDER BY LIMIT optimization is not applied if the inner-most loop can only have a single iteration and is hence not really a loop.
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SHA1:13e3bd3de6b434b6182ef36be108d7ee0be8ca53
User & Date: drh 2016-08-04 09:09:44
Context
2016-08-04
12:35
Add the experimental SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION compile-time option. When enabled, the "unknown function" error is suppressed for EXPLAIN and a no-op function named "unknown()" is substituted. This facilitiates using the command-line shell to analyze queries from applications that contain many application-defined functions that are not normally available to the shell. check-in: b7f30a9f user: drh tags: trunk
09:13
Merge the ORDER BY LIMIT fix, and other enhancements, from trunk. check-in: bf5a9542 user: drh tags: apple-osx
09:09
Make sure the ORDER BY LIMIT optimization is not applied if the inner-most loop can only have a single iteration and is hence not really a loop. check-in: 13e3bd3d user: drh tags: trunk
2016-08-03
14:51
Fix a typo in a comment in btree.c. No changes to code. check-in: 722c1281 user: drh tags: trunk
Changes
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Changes to src/where.c.

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        pWInfo->eDistinct = WHERE_DISTINCT_ORDERED;
      }
    }else{
      pWInfo->nOBSat = pFrom->isOrdered;
      pWInfo->revMask = pFrom->revLoop;
      if( pWInfo->nOBSat<=0 ){
        pWInfo->nOBSat = 0;
        if( nLoop>0 ){
          Bitmask m = 0;
          int rc = wherePathSatisfiesOrderBy(pWInfo, pWInfo->pOrderBy, pFrom,
                      WHERE_ORDERBY_LIMIT, nLoop-1, pFrom->aLoop[nLoop-1], &m);
          if( rc==pWInfo->pOrderBy->nExpr ){
            pWInfo->bOrderedInnerLoop = 1;
            pWInfo->revMask = m;
          }







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        pWInfo->eDistinct = WHERE_DISTINCT_ORDERED;
      }
    }else{
      pWInfo->nOBSat = pFrom->isOrdered;
      pWInfo->revMask = pFrom->revLoop;
      if( pWInfo->nOBSat<=0 ){
        pWInfo->nOBSat = 0;
        if( nLoop>0 && (pFrom->aLoop[nLoop-1]->wsFlags & WHERE_ONEROW)==0 ){
          Bitmask m = 0;
          int rc = wherePathSatisfiesOrderBy(pWInfo, pWInfo->pOrderBy, pFrom,
                      WHERE_ORDERBY_LIMIT, nLoop-1, pFrom->aLoop[nLoop-1], &m);
          if( rc==pWInfo->pOrderBy->nExpr ){
            pWInfo->bOrderedInnerLoop = 1;
            pWInfo->revMask = m;
          }

Changes to test/limit2.test.

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do_execsql_test limit2-120.2 {
  SELECT a, b, '|' FROM t1 WHERE a IN (2,4,5,3,1) ORDER BY +b DESC LIMIT 5;
} {5 9999 | 1 1999 | 1 1998 | 1 1997 | 1 1996 |}
do_test limit2-120.3 {
  set slow_count $sqlite_search_count
  expr {$fast_count < 0.02*$slow_count}
} {1}

























finish_test







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do_execsql_test limit2-120.2 {
  SELECT a, b, '|' FROM t1 WHERE a IN (2,4,5,3,1) ORDER BY +b DESC LIMIT 5;
} {5 9999 | 1 1999 | 1 1998 | 1 1997 | 1 1996 |}
do_test limit2-120.3 {
  set slow_count $sqlite_search_count
  expr {$fast_count < 0.02*$slow_count}
} {1}

# Bug report against the new ORDER BY LIMIT optimization just prior to
# release.  (Unreleased so there is no ticket).
#
# Make sure the optimization is not applied if the inner loop can only
# provide a single row of output.
#
do_execsql_test limit2-200 {
  CREATE TABLE t200(a, b);
  WITH RECURSIVE c(x) AS (VALUES(1) UNION ALL SELECT x+1 FROM c WHERE x<1000)
    INSERT INTO t200(a,b) SELECT x, x FROM c;
  CREATE TABLE t201(x INTEGER PRIMARY KEY, y);
  INSERT INTO t201(x,y) VALUES(2,12345);

  SELECT *, '|' FROM t200, t201 WHERE x=b ORDER BY y LIMIT 3;
} {2 2 2 12345 |}
do_execsql_test limit2-210 {
  SELECT *, '|' FROM t200 LEFT JOIN t201 ON x=b ORDER BY y LIMIT 3;
} {1 1 {} {} | 3 3 {} {} | 4 4 {} {} |}






finish_test