SQLite

/*
** This is the Expr node callback for sqlite3ExprImpliesNotNullRow().
** If the expression node requires that the table at pWalker->iCur
** have a non-NULL column, then set pWalker->eCode to 1 and abort.
*/
static int impliesNotNullRow(Walker *pWalker, Expr *pExpr){
  /* This routine is only called for WHERE clause expressions and so it
  ** cannot have any TK_AGG_COLUMN entries because those are only found
  ** in HAVING clauses.  We can get a TK_AGG_FUNCTION in a WHERE clause,
  ** but that is an illegal construct and the query will be rejected at
  ** a later stage of processing, so the TK_AGG_FUNCTION case does not
  ** need to be considered here. */
  assert( pExpr->op!=TK_AGG_COLUMN );
  testcase( pExpr->op==TK_AGG_FUNCTION );

  if( ExprHasProperty(pExpr, EP_FromJoin) ) return WRC_Prune;
  switch( pExpr->op ){
    case TK_ISNULL:
    case TK_IS:
    case TK_OR:
    case TK_CASE:
    case TK_IN:
    case TK_FUNCTION:
      testcase( pExpr->op==TK_ISNULL );
      testcase( pExpr->op==TK_IS );
      testcase( pExpr->op==TK_OR );
      testcase( pExpr->op==TK_CASE );
      testcase( pExpr->op==TK_IN );
      testcase( pExpr->op==TK_FUNCTION );
      return WRC_Prune;
    case TK_COLUMN:

      if( pWalker->u.iCur==pExpr->iTable ){
        pWalker->eCode = 1;
        return WRC_Abort;
      }
      return WRC_Prune;
    default:
      return WRC_Continue;
  }
}

/*
** Return true (non-zero) if expression p can only be true if at least
** one column of table iTab is non-null.  In other words, return true
** if expression p will always be NULL or false if every column of iTab
** is NULL.
**
** False negatives are acceptable.  In other words, it is ok to return
** zero even if expression p will never be true of every column of iTab
** is NULL.  A false negative is merely a missed optimization opportunity.
**
** False positives are not allowed, however.  A false positive may result
** in an incorrect answer.
**
** Terms of p that are marked with EP_FromJoin (and hence that come from
** the ON or USING clauses of LEFT JOINS) are excluded from the analysis.
**
** This routine is used to check if a LEFT JOIN can be converted into
** an ordinary JOIN.  The p argument is the WHERE clause.  If the WHERE
** clause requires that some column of the right table of the LEFT JOIN