77 *
88 *
99 * IDENTIFICATION
10- * $Header: /cvsroot/pgsql/src/backend/optimizer/path/pathkeys.c,v 1.1 1999/02/2015:27:42 momjian Exp $
10+ * $Header: /cvsroot/pgsql/src/backend/optimizer/path/pathkeys.c,v 1.2 1999/02/2016:28:20 momjian Exp $
1111 *
1212 *-------------------------------------------------------------------------
1313 */
2525#include "optimizer/joininfo.h"
2626#include "optimizer/ordering.h"
2727
28-
2928static int match_pathkey_joinkeys (List * pathkey ,List * joinkeys ,
3029int outer_or_inner );
3130static bool joinkeys_pathkeys_match (List * joinkeys ,List * pathkey ,
@@ -35,6 +34,29 @@ static List *new_join_pathkey(List *subkeys, List *considered_subkeys,
3534static List * new_matching_subkeys (Var * subkey ,List * considered_subkeys ,
3635List * join_rel_tlist ,List * joinclauses );
3736
37+
38+ /*
39+ *Explanation of Path.pathkeys
40+ *
41+ *This structure is a List of List of Var nodes that represent the sort
42+ *order of the result generated by the Path.
43+ *
44+ *In single/base relation RelOptInfo's, the Path's represent various ways
45+ *of generate the relation. Sequential scan Paths have a NIL pathkeys.
46+ *Index scans have Path.pathkeys that represent the chosen index. A
47+ *single-key index pathkeys would be { {tab1_indexkey1} }. The pathkeys
48+ *entry for a multi-key index would be { {tab1_indexkey1}, {tab1_indexkey2} }.
49+ *
50+ *Multi-relation RelOptInfo Path's are more complicated. Mergejoins are
51+ *only performed with equajoins("="). Because of this, the multi-relation
52+ *path actually has more than one ordering. For example, a mergejoin Path
53+ *of "tab1.col1 = tab2.col1" would generate a pathkeys of
54+ *{ {tab1.col1, tab2.col1} }. This allows future joins to use either Var
55+ *as a pre-sorted key to prevent Mergejoins from having to re-sort the Path.
56+ *They are equal, so they are both primary sort keys. This is why pathkeys
57+ *is a List of Lists.
58+ */
59+
3860/****************************************************************************
3961 *KEY COMPARISONS
4062 ****************************************************************************/