----------------------------------------------------------------------- WORKLOG TASK -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- TASK...........: Table elimination CREATION DATE..: Sun, 10 May 2009, 19:57 SUPERVISOR.....: Monty IMPLEMENTOR....: Psergey COPIES TO......: CATEGORY.......: Server-Sprint TASK ID........: 17 (http://askmonty.org/worklog/?tid=17) VERSION........: Server-5.1 STATUS.........: Assigned PRIORITY.......: 60 WORKED HOURS...: 0 ESTIMATE.......: 0 (hours remain) ORIG. ESTIMATE.: 0 PROGRESS NOTES: -=-=(Psergey - Mon, 01 Jun 2009, 20:46)=-=- Low Level Design modified. --- /tmp/wklog.17.old.17448 2009-06-01 20:46:40.000000000 +0300 +++ /tmp/wklog.17.new.17448 2009-06-01 20:46:40.000000000 +0300 @@ -122,3 +122,9 @@ always. If we want table elimination to work in presence of grouping, need to devise some other way of analyzing aggregate functions. + +* Should eliminated tables be shown in EXPLAIN EXTENDED? + - If we just ignore the question, they will be shown + - this is what happens for constant tables, too. + - I don't see how showing them could be of any use. They only make it + harder to read the rewritten query. -=-=(Guest - Mon, 01 Jun 2009, 12:49)=-=- Low Level Design modified. --- /tmp/wklog.17.old.32202 2009-06-01 12:49:15.000000000 +0300 +++ /tmp/wklog.17.new.32202 2009-06-01 12:49:15.000000000 +0300 @@ -8,7 +8,7 @@ 6. Todo, issues to resolve 6.1 To resolve 6.2 Resolved - +7. Additional issues </contents> It's not really about elimination of tables, it's about elimination of inner @@ -116,3 +116,9 @@ * We remove ON clauses within semi-join nests. If these clauses contain subqueries, they probably should be gone from EXPLAIN output also? +* Aggregate functions report they depend on all tables, that is, + + item_agg_func->used_tables() == (1ULL << join->tables) - 1 + + always. If we want table elimination to work in presence of grouping, need + to devise some other way of analyzing aggregate functions. -=-=(Guest - Fri, 29 May 2009, 00:45)=-=- Low Level Design modified. --- /tmp/wklog.17.old.1348 2009-05-29 00:45:21.000000000 +0300 +++ /tmp/wklog.17.new.1348 2009-05-29 00:45:21.000000000 +0300 @@ -111,3 +111,8 @@ referred to an inner table (requirement for OJ->IJ conversion) then table elimination would not be applicable anyway. +7. Additional issues +-------------------- +* We remove ON clauses within semi-join nests. If these clauses contain + subqueries, they probably should be gone from EXPLAIN output also? + -=-=(Guest - Tue, 26 May 2009, 21:52)=-=- Low Level Design modified. --- /tmp/wklog.17.old.14120 2009-05-26 21:52:06.000000000 +0300 +++ /tmp/wklog.17.new.14120 2009-05-26 21:52:06.000000000 +0300 @@ -1,11 +1,14 @@ <contents> 1. Conditions for removal +1.1 Quick check if there are candidates 2. Removal operation properties 3. Removal operation 4. User interface -5. Todo, issues to resolve -5.1 To resolve -5.2 Resolved +5. Tests and benchmarks +6. Todo, issues to resolve +6.1 To resolve +6.2 Resolved + </contents> It's not really about elimination of tables, it's about elimination of inner @@ -29,6 +32,18 @@ GROUP BY and HAVING do not refer to the inner tables of the outer join nest. +1.1 Quick check if there are candidates +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Before we start to enumerate join nests, here is a quick way to check if +there *can be* something to be removed: + + if ((tables used in select_list | + tables used in group/order by UNION | + tables used in where) != bitmap_of_all_tables) + { + attempt table elimination; + } + 2. Removal operation properties ------------------------------- * There is always one way to remove (no choice to remove either this or that) @@ -56,22 +71,24 @@ ----------------- * We'll add an @@optimizer switch flag for table elimination. Tentative name: 'table_elimination'. + (Note ^^ utility of the above questioned ^, as table elimination can never + be worse than no elimination. We're leaning towards not adding the flag) -* With EXPLAIN, there are two options: - - Show removed tables in a way similar to const tables, with some - indication that they are removed. - - Do not show them altogether. -(the second one seems to be better? We're targeting a situation with VIEWs, -where the user would not care about what tables were added into his query -and then discarded from it?) +* EXPLAIN will not show the removed tables at all. This will allow to check + if tables were removed, and also will behave nicely with anchor model and + VIEWs: stuff that user doesn't care about just won't be there. + +5. Tests and benchmarks +----------------------- +Should create a benchmark in sql-bench which checks if the dbms has table +elimination. +TODO elaborate -5. Todo, issues to resolve +6. Todo, issues to resolve -------------------------- -5.1 To resolve +6.1 To resolve ~~~~~~~~~~~~~~ -- See EXPLAIN question in section #4. - - Re-check how this works with equality propagation. - Relationship with prepared statements. @@ -87,7 +104,7 @@ that we'll meet outer joins which have N inner tables of which some are 1-row MyISAM tables that do not have primary key. -5.2 Resolved +6.2 Resolved ~~~~~~~~~~~~ - outer->inner join conversion is not a problem for table elimination. We make outer->inner conversions based on predicates in WHERE. If the WHERE -=-=(Guest - Fri, 22 May 2009, 17:23)=-=- High-Level Specification modified. --- /tmp/wklog.17.old.30851 2009-05-22 17:23:38.000000000 +0300 +++ /tmp/wklog.17.new.30851 2009-05-22 17:23:38.000000000 +0300 @@ -6,7 +6,7 @@ elimination but not to the same extent. Basically, what table elimination does, is to remove tables from the -execution plan when it is unneccessary to include them. This can, of +execution plan when it is unnecessary to include them. This can, of course, only happen if the right circumstances arise. Let us for example look at the following query: @@ -22,30 +22,26 @@ When using A as the left table we ensure that the query will return at least as many rows as there are in that table. For rows where the join condition (B.id = A.id) is not met the selected column (A.colA) will -contain a NULL value. +still contain it's original value. The not seen B.* row would contain all NULL:s. However, the result set could actually contain more rows than what is found in tableA if there are duplicates of the column B.id in tableB. If -A -contains a row [1, "val1"] and B the rows [1, "other1a"],[1, "other1b"] +A contains a row [1, "val1"] and B the rows [1, "other1a"],[1, "other1b"] then two rows will match in the join condition. The only way to know -what -the result will look like is to actually touch both tables during +what the result will look like is to actually touch both tables during execution. Instead, let's say that tableB contains rows that make it possible to place a unique constraint on the column B.id, for example and often the case a primary key. In this situation we know that we will get exactly -as -many rows as there are in tableA, since joining with tableB cannot +as many rows as there are in tableA, since joining with tableB cannot introduce any duplicates. If further, as in the example query, we do not select any columns from tableB, touching that table during execution is -unneccessary. We can remove the whole join operation from the execution +unnecessary. We can remove the whole join operation from the execution plan. Both SQL Server 2005/2008 and Oracle 11g will deploy table elimination -in -the case described above. Let us look at a more advanced query, where +in the case described above. Let us look at a more advanced query, where Oracle fails. select -=-=(Guest - Fri, 22 May 2009, 17:00)=-=- Version updated. --- /tmp/wklog.17.old.30176 2009-05-22 17:00:35.000000000 +0300 +++ /tmp/wklog.17.new.30176 2009-05-22 17:00:35.000000000 +0300 @@ -1 +1 @@ -Maria-2.0 +Server-5.1 -=-=(Guest - Fri, 22 May 2009, 17:00)=-=- Category updated. --- /tmp/wklog.17.old.30162 2009-05-22 17:00:28.000000000 +0300 +++ /tmp/wklog.17.new.30162 2009-05-22 17:00:28.000000000 +0300 @@ -1 +1 @@ -Maria-Sprint +Server-Sprint -=-=(Guest - Fri, 22 May 2009, 17:00)=-=- Version updated. --- /tmp/wklog.17.old.30162 2009-05-22 17:00:28.000000000 +0300 +++ /tmp/wklog.17.new.30162 2009-05-22 17:00:28.000000000 +0300 @@ -1 +1 @@ -2.0 +9.x -=-=(Guest - Fri, 22 May 2009, 17:00)=-=- Version updated. --- /tmp/wklog.17.old.30162 2009-05-22 17:00:28.000000000 +0300 +++ /tmp/wklog.17.new.30162 2009-05-22 17:00:28.000000000 +0300 @@ -1 +1 @@ -9.x +Maria-2.0 -=-=(Guest - Fri, 22 May 2009, 17:00)=-=- Category updated. --- /tmp/wklog.17.old.30122 2009-05-22 17:00:02.000000000 +0300 +++ /tmp/wklog.17.new.30122 2009-05-22 17:00:02.000000000 +0300 @@ -1 +1 @@ -Server-RawIdeaBin +Maria-Sprint ------------------------------------------------------------ -=-=(View All Progress Notes, 17 total)=-=- http://askmonty.org/worklog/index.pl?tid=17&nolimit=1 DESCRIPTION: Eliminate not needed tables from SELECT queries.. This will speed up some views and automatically generated queries. Example: CREATE TABLE B (id int primary key); select A.colA from tableA A left outer join tableB B on B.id = A.id; In this case we can remove table B and the join from the query. HIGH-LEVEL SPECIFICATION: Here is an extended explanation of table elimination. Table elimination is a feature found in some modern query optimizers, of which Microsoft SQL Server 2005/2008 seems to have the most advanced implementation. Oracle 11g has also been confirmed to use table elimination but not to the same extent. Basically, what table elimination does, is to remove tables from the execution plan when it is unnecessary to include them. This can, of course, only happen if the right circumstances arise. Let us for example look at the following query: select A.colA from tableA A left outer join tableB B on B.id = A.id; When using A as the left table we ensure that the query will return at least as many rows as there are in that table. For rows where the join condition (B.id = A.id) is not met the selected column (A.colA) will still contain it's original value. The not seen B.* row would contain all NULL:s. However, the result set could actually contain more rows than what is found in tableA if there are duplicates of the column B.id in tableB. If A contains a row [1, "val1"] and B the rows [1, "other1a"],[1, "other1b"] then two rows will match in the join condition. The only way to know what the result will look like is to actually touch both tables during execution. Instead, let's say that tableB contains rows that make it possible to place a unique constraint on the column B.id, for example and often the case a primary key. In this situation we know that we will get exactly as many rows as there are in tableA, since joining with tableB cannot introduce any duplicates. If further, as in the example query, we do not select any columns from tableB, touching that table during execution is unnecessary. We can remove the whole join operation from the execution plan. Both SQL Server 2005/2008 and Oracle 11g will deploy table elimination in the case described above. Let us look at a more advanced query, where Oracle fails. select A.colA from tableA A left outer join tableB B on B.id = A.id and B.fromDate = ( select max(sub.fromDate) from tableB sub where sub.id = A.id ); In this example we have added another join condition, which ensures that we only pick the matching row from tableB having the latest fromDate. In this case tableB will contain duplicates of the column B.id, so in order to ensure uniqueness the primary key has to contain the fromDate column as well. In other words the primary key of tableB is (B.id, B.fromDate). Furthermore, since the subselect ensures that we only pick the latest B.fromDate for a given B.id we know that at most one row will match the join condition. We will again have the situation where joining with tableB cannot affect the number of rows in the result set. Since we do not select any columns from tableB, the whole join operation can be eliminated from the execution plan. SQL Server 2005/2008 will deploy table elimination in this situation as well. We have not found a way to make Oracle 11g use it for this type of query. Queries like these arise in two situations. Either when you have denormalized model consisting of a fact table with several related dimension tables, or when you have a highly normalized model where each attribute is stored in its own table. The example with the subselect is common whenever you store historized/versioned data. LOW-LEVEL DESIGN: <contents> 1. Conditions for removal 1.1 Quick check if there are candidates 2. Removal operation properties 3. Removal operation 4. User interface 5. Tests and benchmarks 6. Todo, issues to resolve 6.1 To resolve 6.2 Resolved 7. Additional issues </contents> It's not really about elimination of tables, it's about elimination of inner sides of outer joins. 1. Conditions for removal ------------------------- We can eliminate an inner side of outer join if: 1. For each record combination of outer tables, it will always produce exactly one record. 2. There are no references to columns of the inner tables anywhere else in the query. #1 means that every table inside the outer join nest is: - is a constant table: = because it can be accessed via eq_ref(const) access, or = it is a zero-rows or one-row MyISAM-like table [MARK1] - has an eq_ref access method candidate. #2 means that WHERE clause, ON clauses of embedding outer joins, ORDER BY, GROUP BY and HAVING do not refer to the inner tables of the outer join nest. 1.1 Quick check if there are candidates ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Before we start to enumerate join nests, here is a quick way to check if there *can be* something to be removed: if ((tables used in select_list | tables used in group/order by UNION | tables used in where) != bitmap_of_all_tables) { attempt table elimination; } 2. Removal operation properties ------------------------------- * There is always one way to remove (no choice to remove either this or that) * It is always better to remove as much tables as possible (at least within our cost model). Thus, no need for any cost calculations/etc. It's an unconditional rewrite. 3. Removal operation -------------------- * Remove the outer join nest's nested join structure (i.e. get the outer join's TABLE_LIST object $OJ and remove it from $OJ->embedding, $OJ->embedding->nested_join. Update table_map's of all ancestor nested joins). [MARK2] * Move the tables and their JOIN_TABs to front like it is done with const tables, with exception that if eliminated outer join nest was within another outer join nest, that shouldn't prevent us from moving away the eliminated tables. * Update join->table_count and all-join-tables bitmap. * That's it. Nothing else? 4. User interface ----------------- * We'll add an @@optimizer switch flag for table elimination. Tentative name: 'table_elimination'. (Note ^^ utility of the above questioned ^, as table elimination can never be worse than no elimination. We're leaning towards not adding the flag) * EXPLAIN will not show the removed tables at all. This will allow to check if tables were removed, and also will behave nicely with anchor model and VIEWs: stuff that user doesn't care about just won't be there. 5. Tests and benchmarks ----------------------- Should create a benchmark in sql-bench which checks if the dbms has table elimination. TODO elaborate 6. Todo, issues to resolve -------------------------- 6.1 To resolve ~~~~~~~~~~~~~~ - Re-check how this works with equality propagation. - Relationship with prepared statements. On one hand, it's natural to desire to make table elimination a once-per-statement operation, like outer->inner join conversion. We'll have to limit the applicability by removing [MARK1] as that can change during lifetime of the statement. The other option is to do table elimination every time. This will require to rework operation [MARK2] to be undoable. I'm leaning towards doing the former. With anchor modeling, it is unlikely that we'll meet outer joins which have N inner tables of which some are 1-row MyISAM tables that do not have primary key. 6.2 Resolved ~~~~~~~~~~~~ - outer->inner join conversion is not a problem for table elimination. We make outer->inner conversions based on predicates in WHERE. If the WHERE referred to an inner table (requirement for OJ->IJ conversion) then table elimination would not be applicable anyway. 7. Additional issues -------------------- * We remove ON clauses within semi-join nests. If these clauses contain subqueries, they probably should be gone from EXPLAIN output also? * Aggregate functions report they depend on all tables, that is, item_agg_func->used_tables() == (1ULL << join->tables) - 1 always. If we want table elimination to work in presence of grouping, need to devise some other way of analyzing aggregate functions. * Should eliminated tables be shown in EXPLAIN EXTENDED? - If we just ignore the question, they will be shown - this is what happens for constant tables, too. - I don't see how showing them could be of any use. They only make it harder to read the rewritten query. ESTIMATED WORK TIME ESTIMATED COMPLETION DATE ----------------------------------------------------------------------- WorkLog (v3.5.9)