diff options
Diffstat (limited to 'src/backend')
| -rw-r--r-- | src/backend/optimizer/path/pathkeys.c | 26 |
1 files changed, 24 insertions, 2 deletions
diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c index 996c197350c..29f11dcd1c4 100644 --- a/src/backend/optimizer/path/pathkeys.c +++ b/src/backend/optimizer/path/pathkeys.c @@ -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 ****************************************************************************/ |