diff options
Diffstat (limited to 'src/include/nodes/parsenodes.h')
| -rw-r--r-- | src/include/nodes/parsenodes.h | 17 |
1 files changed, 11 insertions, 6 deletions
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h index e6de4b49c85..fa78b4b188b 100644 --- a/src/include/nodes/parsenodes.h +++ b/src/include/nodes/parsenodes.h @@ -7,7 +7,7 @@ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/include/nodes/parsenodes.h,v 1.368 2008/07/18 03:32:53 tgl Exp $ + * $PostgreSQL: pgsql/src/include/nodes/parsenodes.h,v 1.369 2008/07/31 22:47:56 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -613,14 +613,19 @@ typedef struct RangeTblEntry * tleSortGroupRef must match ressortgroupref of exactly one entry of the * associated targetlist; that is the expression to be sorted (or grouped) by. * sortop is the OID of the ordering operator (a "<" or ">" operator). - * nulls_first does about what you'd expect. + * nulls_first means about what you'd expect. * * SortClauses are also used to identify targets that we will do a "Unique" * filter step on (for SELECT DISTINCT and SELECT DISTINCT ON). The - * distinctClause list is simply a copy of the relevant members of the - * sortClause list. Note that distinctClause can be a subset of sortClause, - * but cannot have members not present in sortClause; and the members that - * do appear must be in the same order as in sortClause. + * distinctClause list is a list of SortClauses for the expressions to be + * unique-ified. (As per comment for GroupClause, this overspecifies the + * semantics.) In SELECT DISTINCT, the distinctClause list is typically + * longer than the ORDER BY list, while in SELECT DISTINCT ON it's typically + * shorter. The two lists must match up to the end of the shorter one --- + * the parser rearranges the distinctClause if necessary to make this true. + * (This restriction ensures that only one sort step is needed to both + * satisfy the ORDER BY and set up for the Unique step. This is semantically + * necessary for DISTINCT ON, and offers no real drawback for DISTINCT.) */ typedef struct SortClause { |