diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/select.c | 19 |
1 files changed, 12 insertions, 7 deletions
diff --git a/src/select.c b/src/select.c index f0f4b6a5f..188050e8d 100644 --- a/src/select.c +++ b/src/select.c @@ -2588,9 +2588,8 @@ static void substSelect( #if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW) /* -** This routine attempts to flatten subqueries in order to speed -** execution. It returns 1 if it makes changes and 0 if no flattening -** occurs. +** This routine attempts to flatten subqueries as a performance optimization. +** This routine returns 1 if it makes changes and 0 if no flattening occurs. ** ** To understand the concept of flattening, consider the following ** query: @@ -2632,7 +2631,10 @@ static void substSelect( ** (6) The subquery does not use aggregates or the outer query is not ** DISTINCT. ** -** (7) The subquery has a FROM clause. +** (7) The subquery has a FROM clause. TODO: For subqueries without +** A FROM clause, consider adding a FROM close with the special +** table sqlite_once that consists of a single row containing a +** single NULL. ** ** (8) The subquery does not use LIMIT or the outer query is not a join. ** @@ -2665,11 +2667,14 @@ static void substSelect( ** ** * is not itself part of a compound select, ** * is not an aggregate or DISTINCT query, and -** * has no other tables or sub-selects in the FROM clause. +** * is not a join ** ** The parent and sub-query may contain WHERE clauses. Subject to ** rules (11), (13) and (14), they may also contain ORDER BY, -** LIMIT and OFFSET clauses. +** LIMIT and OFFSET clauses. The subquery cannot use any compound +** operator other than UNION ALL because all the other compound +** operators have an implied DISTINCT which is disallowed by +** restriction (4). ** ** (18) If the sub-query is a compound select, then all terms of the ** ORDER by clause of the parent must be simple references to @@ -2681,7 +2686,7 @@ static void substSelect( ** (20) If the sub-query is a compound select, then it must not use ** an ORDER BY clause. Ticket #3773. We could relax this constraint ** somewhat by saying that the terms of the ORDER BY clause must -** appear as unmodified result columns in the outer query. But +** appear as unmodified result columns in the outer query. But we ** have other optimizations in mind to deal with that case. ** ** (21) The subquery does not use LIMIT or the outer query is not |