diff options
Diffstat (limited to 'src/backend/optimizer/path/joinrels.c')
| -rw-r--r-- | src/backend/optimizer/path/joinrels.c | 21 |
1 files changed, 19 insertions, 2 deletions
diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c index 1ac9a3064b3..26ff2e57586 100644 --- a/src/backend/optimizer/path/joinrels.c +++ b/src/backend/optimizer/path/joinrels.c @@ -8,7 +8,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.64 2003/11/29 19:51:50 pgsql Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.65 2003/12/17 17:07:48 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -226,7 +226,24 @@ make_rels_by_joins(Query *root, int level, List **joinrels) } } - if (result_rels == NIL) + /*---------- + * When IN clauses are involved, there may be no legal way to make + * an N-way join for some values of N. For example consider + * + * SELECT ... FROM t1 WHERE + * x IN (SELECT ... FROM t2,t3 WHERE ...) AND + * y IN (SELECT ... FROM t4,t5 WHERE ...) + * + * We will flatten this query to a 5-way join problem, but there are + * no 4-way joins that make_join_rel() will consider legal. We have + * to accept failure at level 4 and go on to discover a workable + * bushy plan at level 5. + * + * However, if there are no IN clauses then make_join_rel() should + * never fail, and so the following sanity check is useful. + *---------- + */ + if (result_rels == NIL && root->in_info_list == NIL) elog(ERROR, "failed to build any %d-way joins", level); } |