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Overview
Comment:Prefer to use partial indexes for full table scans when that is possible.
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SHA1: fe1874321ba31cec9ae65387920c33d8d0178ed8
User & Date: drh 2016-06-08 18:07:21
References
2019-05-04
16:59 New ticket [5c695520] Incorrect result on a table scan of a partial index. artifact: d567b0d2 user: drh
Context
2016-06-10
22:49
Enhance "PRAGMA table_info" to that it provides information about eponymous virtual tables. check-in: 53a1e5d5 user: drh tags: trunk
2016-06-08
18:07
Prefer to use partial indexes for full table scans when that is possible. check-in: fe187432 user: drh tags: trunk
14:04
Add the "dbhash.exe" utility program that computes a SHA1 hash over the invariant content of an SQLite database file. Free space in the file, the page size, auto_vacuum status, text encoding, and so forth do not change the hash. Only the content matters. check-in: f48a4ad3 user: drh tags: trunk
Changes
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Changes to src/build.c.

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  /*                10,  9,  8,  7,  6 */
  LogEst aVal[] = { 33, 32, 30, 28, 26 };
  LogEst *a = pIdx->aiRowLogEst;
  int nCopy = MIN(ArraySize(aVal), pIdx->nKeyCol);
  int i;

  /* Set the first entry (number of rows in the index) to the estimated 
  ** number of rows in the table. Or 10, if the estimated number of rows 
  ** in the table is less than that.  */
  a[0] = pIdx->pTable->nRowLogEst;

  if( a[0]<33 ) a[0] = 33;        assert( 33==sqlite3LogEst(10) );

  /* Estimate that a[1] is 10, a[2] is 9, a[3] is 8, a[4] is 7, a[5] is
  ** 6 and each subsequent value (if any) is 5.  */
  memcpy(&a[1], aVal, nCopy*sizeof(LogEst));
  for(i=nCopy+1; i<=pIdx->nKeyCol; i++){
    a[i] = 23;                    assert( 23==sqlite3LogEst(5) );
  }







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  /*                10,  9,  8,  7,  6 */
  LogEst aVal[] = { 33, 32, 30, 28, 26 };
  LogEst *a = pIdx->aiRowLogEst;
  int nCopy = MIN(ArraySize(aVal), pIdx->nKeyCol);
  int i;

  /* Set the first entry (number of rows in the index) to the estimated 
  ** number of rows in the table, or half the number of rows in the table
  ** for a partial index.   But do not let the estimate drop below 10. */
  a[0] = pIdx->pTable->nRowLogEst;
  if( pIdx->pPartIdxWhere!=0 ) a[0] -= 10;  assert( 10==sqlite3LogEst(2) );
  if( a[0]<33 ) a[0] = 33;                  assert( 33==sqlite3LogEst(10) );

  /* Estimate that a[1] is 10, a[2] is 9, a[3] is 8, a[4] is 7, a[5] is
  ** 6 and each subsequent value (if any) is 5.  */
  memcpy(&a[1], aVal, nCopy*sizeof(LogEst));
  for(i=nCopy+1; i<=pIdx->nKeyCol; i++){
    a[i] = 23;                    assert( 23==sqlite3LogEst(5) );
  }

Changes to src/where.c.

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        m = pSrc->colUsed & ~columnsInIndex(pProbe);
        pNew->wsFlags = (m==0) ? (WHERE_IDX_ONLY|WHERE_INDEXED) : WHERE_INDEXED;
      }

      /* Full scan via index */
      if( b
       || !HasRowid(pTab)

       || ( m==0
         && pProbe->bUnordered==0
         && (pProbe->szIdxRow<pTab->szTabRow)
         && (pWInfo->wctrlFlags & WHERE_ONEPASS_DESIRED)==0
         && sqlite3GlobalConfig.bUseCis
         && OptimizationEnabled(pWInfo->pParse->db, SQLITE_CoverIdxScan)
          )







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        m = pSrc->colUsed & ~columnsInIndex(pProbe);
        pNew->wsFlags = (m==0) ? (WHERE_IDX_ONLY|WHERE_INDEXED) : WHERE_INDEXED;
      }

      /* Full scan via index */
      if( b
       || !HasRowid(pTab)
       || pProbe->pPartIdxWhere!=0
       || ( m==0
         && pProbe->bUnordered==0
         && (pProbe->szIdxRow<pTab->szTabRow)
         && (pWInfo->wctrlFlags & WHERE_ONEPASS_DESIRED)==0
         && sqlite3GlobalConfig.bUseCis
         && OptimizationEnabled(pWInfo->pParse->db, SQLITE_CoverIdxScan)
          )

Changes to test/index6.test.

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do_execsql_test index6-10.3 {
  SELECT e FROM t10 WHERE a=1 AND b=2 ORDER BY d DESC;
} {9 5}
do_execsql_test index6-10.3eqp {
  EXPLAIN QUERY PLAN
  SELECT e FROM t10 WHERE a=1 AND b=2 ORDER BY d DESC;
} {~/USING INDEX t10x/}












finish_test








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do_execsql_test index6-10.3 {
  SELECT e FROM t10 WHERE a=1 AND b=2 ORDER BY d DESC;
} {9 5}
do_execsql_test index6-10.3eqp {
  EXPLAIN QUERY PLAN
  SELECT e FROM t10 WHERE a=1 AND b=2 ORDER BY d DESC;
} {~/USING INDEX t10x/}

# A partial index will be used for a full table scan, where possible
do_execsql_test index6-11.1 {
  CREATE TABLE t11(a,b,c);
  CREATE INDEX t11x ON t11(a) WHERE b<>99;
  EXPLAIN QUERY PLAN SELECT a FROM t11 WHERE b<>99;
} {/USING INDEX t11x/}
do_execsql_test index6-11.2 {
  EXPLAIN QUERY PLAN SELECT a FROM t11 WHERE b<>99 AND c<>98;
} {/USING INDEX t11x/}
  

finish_test