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Comment changes in select.c. (CVS 4691)

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FossilOrigin-Name: 38020592f15c072e0d221ae2e0df13508ac4bd49
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danielk1977
2008-01-07 10:16:40 +00:00
parent 1013c9320d
commit 738bdcfbf8
3 changed files with 38 additions and 11 deletions
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33
src/select.c
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@@ -12,7 +12,7 @@
** This file contains C code routines that are called by the parser
** to handle SELECT statements in SQLite.
**
** $Id: select.c,v 1.390 2008/01/06 00:25:22 drh Exp $
** $Id: select.c,v 1.391 2008/01/07 10:16:41 danielk1977 Exp $
*/
#include "sqliteInt.h"
@@ -2642,8 +2642,10 @@ static int flattenSubquery(
** it is, or 0 otherwise. At present, a query is considered to be
** a min()/max() query if:
**
** 1. The result set contains exactly one element, either
** min(x) or max(x), where x is a column identifier.
** 1. There is a single object in the FROM clause.
**
** 2. There is a single expression in the result set, and it is
** either min(x) or max(x), where x is a column reference.
*/
static int minMaxQuery(Parse *pParse, Select *p){
Expr *pExpr;
@@ -3652,6 +3654,31 @@ int sqlite3Select(
ExprList *pDel = 0;
u8 flag;
/* Check if the query is of one of the following forms:
**
** SELECT min(x) FROM ...
** SELECT max(x) FROM ...
**
** If it is, then ask the code in where.c to attempt to sort results
** as if there was an "ORDER ON x" or "ORDER ON x DESC" clause.
** If where.c is able to produce results sorted in this order, then
** add vdbe code to break out of the processing loop after the
** first iteration (since the first iteration of the loop is
** guaranteed to operate on the row with the minimum or maximum
** value of x, the only row required).
**
** A special flag must be passed to sqlite3WhereBegin() to slightly
** modify behaviour as follows:
**
** + If the query is a "SELECT min(x)", then the loop coded by
** where.c should not iterate over any values with a NULL value
** for x.
**
** + The optimizer code in where.c (the thing that decides which
** index or indices to use) should place a different priority on
** satisfying the 'ORDER BY' clause than it does in other cases.
** Refer to code and comments in where.c for details.
*/
flag = minMaxQuery(pParse, p);
if( flag ){
pMinMax = sqlite3ExprListDup(db, p->pEList->a[0].pExpr->pList);