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add pathkeys description.

This commit is contained in:
Bruce Momjian 1999-02-20 16:28:20 +00:00
parent 67fd67f53a
commit 9188aab758
2 changed files with 29 additions and 6 deletions
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26
src/backend/optimizer/path/pathkeys.c
View File

@ -7,7 +7,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/pathkeys.c,v 1.1 1999/02/20 15:27:42 momjian Exp $
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/pathkeys.c,v 1.2 1999/02/20 16:28:20 momjian Exp $
*
*-------------------------------------------------------------------------
*/
@ -25,7 +25,6 @@
#include "optimizer/joininfo.h"
#include "optimizer/ordering.h"
static int match_pathkey_joinkeys(List *pathkey, List *joinkeys,
int outer_or_inner);
static bool joinkeys_pathkeys_match(List *joinkeys, List *pathkey,
@ -35,6 +34,29 @@ static List *new_join_pathkey(List *subkeys, List *considered_subkeys,
static List *new_matching_subkeys(Var *subkey, List *considered_subkeys,
List *join_rel_tlist, List *joinclauses);
/*
* Explanation of Path.pathkeys
*
* This structure is a List of List of Var nodes that represent the sort
* order of the result generated by the Path.
*
* In single/base relation RelOptInfo's, the Path's represent various ways
* of generate the relation. Sequential scan Paths have a NIL pathkeys.
* Index scans have Path.pathkeys that represent the chosen index. A
* single-key index pathkeys would be { {tab1_indexkey1} }. The pathkeys
* entry for a multi-key index would be { {tab1_indexkey1}, {tab1_indexkey2} }.
*
* Multi-relation RelOptInfo Path's are more complicated. Mergejoins are
* only performed with equajoins("="). Because of this, the multi-relation
* path actually has more than one ordering. For example, a mergejoin Path
* of "tab1.col1 = tab2.col1" would generate a pathkeys of
* { {tab1.col1, tab2.col1} }. This allows future joins to use either Var
* as a pre-sorted key to prevent Mergejoins from having to re-sort the Path.
* They are equal, so they are both primary sort keys. This is why pathkeys
* is a List of Lists.
*/
/****************************************************************************
* KEY COMPARISONS
****************************************************************************/

9
src/include/nodes/relation.h
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@ -6,7 +6,7 @@
*
* Copyright (c) 1994, Regents of the University of California
*
* $Id: relation.h,v 1.26 1999/02/18 00:49:38 momjian Exp $
* $Id: relation.h,v 1.27 1999/02/20 16:28:20 momjian Exp $
*
*-------------------------------------------------------------------------
*/
@ -142,9 +142,10 @@ typedef struct Path
PathOrder *pathorder;
List *pathkeys; /* This is a List of List of Var nodes.
* It is a List of Lists because of multi-key
* indexes.
List *pathkeys; /*
* This is a List of List of Var nodes.
* See the top of optimizer/path/pathkeys.c
* for more information.
*/
Cost outerjoincost;
Relids joinid;