SQLite

    endOfLoop = sqlite3VdbeMakeLabel(v);
    sqlite3VdbeAddOp2(v, OP_Rewind, iIdxCur, endOfLoop);
    topOfLoop = sqlite3VdbeCurrentAddr(v);
    sqlite3VdbeAddOp2(v, OP_AddImm, iMem, 1);

    for(i=0; i<nCol; i++){
      sqlite3VdbeAddOp3(v, OP_Column, iIdxCur, i, regCol);

      if( i==0 ){
#ifdef SQLITE_ENABLE_STAT2
        /* Check if the record that cursor iIdxCur points to contains a
        ** value that should be stored in the sqlite_stat2 table. If so,
        ** store it.  */
        int ne = sqlite3VdbeAddOp3(v, OP_Ne, regRecno, 0, regSamplerecno);
        assert( regTabname+1==regIdxname 
             && regTabname+2==regSampleno
             && regTabname+3==regCol

        sqlite3VdbeAddOp2(v, OP_Integer, SQLITE_INDEX_SAMPLES, regTemp2);
        sqlite3VdbeAddOp3(v, OP_Subtract, regSampleno, regTemp2, regTemp2);
        sqlite3VdbeAddOp3(v, OP_Divide, regTemp2, regTemp, regTemp);
        sqlite3VdbeAddOp3(v, OP_Add, regSamplerecno, regTemp, regSamplerecno);

        sqlite3VdbeJumpHere(v, ne);
        sqlite3VdbeAddOp2(v, OP_AddImm, regRecno, 1);

#endif

        /* Always record the very first row */
        sqlite3VdbeAddOp1(v, OP_IfNot, iMem+1);
      }
      sqlite3VdbeAddOp3(v, OP_Ne, regCol, 0, iMem+nCol+i+1);
      /**** TODO:  add collating sequence *****/
      sqlite3VdbeChangeP5(v, SQLITE_NULLEQ);
    }
    if( db->mallocFailed ){
      /* If a malloc failure has occurred, then the result of the expression 
      ** passed as the second argument to the call to sqlite3VdbeJumpHere() 
      ** below may be negative. Which causes an assert() to fail (or an
      ** out-of-bounds write if SQLITE_DEBUG is not defined).  */
      return;
    }
    sqlite3VdbeAddOp2(v, OP_Goto, 0, endOfLoop);
    for(i=0; i<nCol; i++){
      int addr = sqlite3VdbeCurrentAddr(v) - (nCol*2);
      if( i==0 ){
        sqlite3VdbeJumpHere(v, addr-1);  /* Set jump dest for the OP_IfNot */
      }
      sqlite3VdbeJumpHere(v, addr);      /* Set jump dest for the OP_Ne */
      sqlite3VdbeAddOp2(v, OP_AddImm, iMem+i+1, 1);
      sqlite3VdbeAddOp3(v, OP_Column, iIdxCur, i, iMem+nCol+i+1);
    }

    /* End of the analysis loop. */
    sqlite3VdbeResolveLabel(v, endOfLoop);
    sqlite3VdbeAddOp2(v, OP_Next, iIdxCur, topOfLoop);

# 2011 January 04
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
#
# This file implements regression tests for SQLite library. This file 
# implements tests for ANALYZE to verify that multiple rows containing
# a NULL value count as distinct rows for the purposes of analyze 
# statistics.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl

do_test analyze4-1.0 {
  db eval {
    CREATE TABLE t1(a,b);
    CREATE INDEX t1a ON t1(a);
    CREATE INDEX t1b ON t1(b);
    INSERT INTO t1 VALUES(1,NULL);
    INSERT INTO t1 SELECT a+1, b FROM t1;
    INSERT INTO t1 SELECT a+2, b FROM t1;
    INSERT INTO t1 SELECT a+4, b FROM t1;
    INSERT INTO t1 SELECT a+8, b FROM t1;
    INSERT INTO t1 SELECT a+16, b FROM t1;
    INSERT INTO t1 SELECT a+32, b FROM t1;
    INSERT INTO t1 SELECT a+64, b FROM t1;
    ANALYZE;
  }

  # Should choose the t1a index since it is more specific than t1b.
  db eval {EXPLAIN QUERY PLAN SELECT * FROM t1 WHERE a=5 AND b IS NULL}
} {0 0 0 {SEARCH TABLE t1 USING INDEX t1a (a=?) (~1 rows)}}

# Verify that the t1b index shows that it does not narrow down the
# search any at all.
#
do_test analyze4-1.1 {
  db eval {
    SELECT idx, stat FROM sqlite_stat1 WHERE tbl='t1' ORDER BY idx;
  }
} {t1a {128 1} t1b {128 128}}

# Change half of the b values from NULL to a constant.  Verify
# that the number of rows selected in stat1 is half the total 
# number of rows.
#
do_test analyze4-1.2 {
  db eval {
    UPDATE t1 SET b='x' WHERE a%2;
    ANALYZE;
    SELECT idx, stat FROM sqlite_stat1 WHERE tbl='t1' ORDER BY idx;
  }
} {t1a {128 1} t1b {128 64}}

# Change the t1.b values all back to NULL.  Add columns t1.c and t1.d.
# Create a multi-column indices using t1.b and verify that ANALYZE 
# processes them correctly.
#
do_test analyze4-1.3 {
  db eval {
    UPDATE t1 SET b=NULL;
    ALTER TABLE t1 ADD COLUMN c;
    ALTER TABLE t1 ADD COLUMN d;
    UPDATE t1 SET c=a/4, d=a/2;
    CREATE INDEX t1bcd ON t1(b,c,d);
    CREATE INDEX t1cdb ON t1(c,d,b);
    CREATE INDEX t1cbd ON t1(c,b,d);
    ANALYZE;
    SELECT idx, stat FROM sqlite_stat1 WHERE tbl='t1' ORDER BY idx;
  }
} {t1a {128 1} t1b {128 128} t1bcd {128 128 4 2} t1cbd {128 4 4 2} t1cdb {128 4 2 2}}

finish_test