This bug led to wrong result sets returned by the second execution of
prepared statements from selects using mergeable derived tables pushed
into external engine. Such derived tables are always materialized. The
decision that they have to be materialized is taken late in the function
mysql_derived_optimized(). For regular derived tables this decision is
usually taken at the prepare phase. However in some cases for some derived
tables this decision is made in mysql_derived_optimized() too. It can be
seen in the code of mysql_derived_fill() that for such a derived table it's
critical to change its translation table to tune it to the fields of the
temporary table used for materialization of the derived table and this
must be done after each refill of the derived table. The same actions are
needed for derived tables pushed into external engines.
Approved by Oleksandr Byelkin <sanja@mariadb.com>
Pushing down statements to FederatedX engine is implemented by
printing either SELECT_LEX or SELECT_LEX_UNIT into a string and
sending that string to the engine. In the case of pushing down a
single SELECT having a CTE (WITH clause) there was a problem, because
normally single SELECTs were printed using SELECT_LEX::print().
But CTEs are stored in the upper unit of the SELECT_LEX -
SELECT_LEX_UNIT, so they were not unfolded in the string produced.
The solution is to invoke SELECT_LEX_UNIT::print() when pushing down
single SELECT statements (but not those which are parts of units),
so the possible CTEs are unfolded and printed.
Reviewed by Sergei Petrunia (sergey@mariadb.com)
ANALYZE FORMAT=JSON output now includes table.r_engine_stats which
has the engine statistics. Only non-zero members are printed.
Internally: EXPLAIN data structures Explain_table_acccess and
Explain_update now have handler* handler_for_stats pointer.
It is used to read statistics from handler_for_stats->handler_stats.
The following applies only to 10.9+, backport doesn't use it:
Explain data structures exist after the tables are closed. We avoid
walking invalid pointers using this:
- SQL layer calls Explain_query::notify_tables_are_closed() before
closing tables.
- After that call, printing of JSON output is disabled. Non-JSON output
can be printed but we don't access handler_for_stats when doing that.
There was no actual execution of the SQL of a pushed derived table,
which caused "r_rows" to be always displayed as 0 and "r_total_time_ms"
to show inaccurate numbers.
This commit makes a derived table SQL to be executed by the storage
engine, so the server is able to calculate the number of rows returned
and measure the execution time more accurately
Fake_select_lex->join was prepared at the unit execution stage so
the validation of fake_select_lex before the unit pushdown
was incomplete. That caused pushing down of statements having
an incorrect ORDER BY clause.
This commit moves preparation of the fake_select_lex->join to the unit
prepare() method, before initializing of the pushdown handler,
so incorrect clauses error out before being pushed down
During st_select_lex_unit::prepare() the member select_unit*
st_select_lex_unit::union_result is being assigned to an instance
of one of the following classes:
- select_unit
- select_unit_ext
- select_unit_recursive
- select_union_direct
Select_union_direct used to pass the result of the query directly to
the receiving select_result without filling a temporary table. This class
wraps a select_result object and is currently used to process UNION ALL
queries. Other select_unit_* classes involve some additional result processing.
Pushed down units are processed on the engine side so the results must be
also passed directly to a select_result object. So in the case when
the unit pushdown is employed st_select_lex_unit::union_result must be
assigned to an instance of select_union_direct.
Allow queries of multiple SELECTs combined together with
UNIONs/EXCEPTs/INTERSECTs to be pushed down to foreign engines.
If the foreign engine provides an interface method "create_unit"
and the UNIT is a top-level unit of the SQL query then the server
tries to push the whole SELECT_LEX_UNIT down to the engine for execution.
The engine should perform necessary checks and if they succeed,
execute the query. If the engine is unable to execute the whole unit,
then another attempt is made to push down SELECTs composing the unit
separately using the "create_select" interface method. In this case
the results of separate SELECTs are combined at the server side
thus composing the final result
The problem was the mysql_derived_prepare() did not correctly set
'distinct' when creating a temporary derivated table.
Fixed by separating checking for distinct for queries with and without
UNION.
Other things:
- Fixed bug in generate_derived_keys_for_table() where we set the wrong
bit for join_tab->keys
- Cleaned up JOIN::drop_unused_derived_keys()
- Changed TABLE::use_index() to keep unique keys and update
share->key_parts
Author: Sergei Petrunia <sergey@mariadb.com>, monty@mariadb.org
The main difference in code path between EQ_REF and REF is that for
REF we have to do an extra read_next on the index to check that there
is no more matching rows.
Before this patch we added a preference of EQ_REF by ensuring that REF
would always estimate to find at least 2 rows.
This patch adds the cost of the extra key read_next to REF access and
removes the code that limited REF to at least 2 rows. For some queries
this can have a big effect as the total estimated rows will be halved
for each REF table with 1 rows.
multi_range cost calculations are also changed to take into account
the difference between EQ_REF and REF.
The effect of the patch to the test suite:
- About 80 test case changed
- Almost all changes where for EXPLAIN where estimated rows for REF
where changed from 2 to 1.
- A few test cases using explain extended had a change of 'filtered'.
This is because of the estimated rows are now closer to the
calculated selectivity.
- A very few test had a change of table order.
This is because the change of estimated rows from 2 to 1 or the small
cost change for REF
(main.subselect_sj_jcl6, main.group_by, main.dervied_cond_pushdown,
main.distinct, main.join_nested, main.order_by, main.join_cache)
- No key statistics and the estimated rows are now smaller which cased
estimated filtering to be lower.
(main.subselect_sj_mat)
- The number of total rows are halved.
(main.derived_cond_pushdown)
- Plans with 1 row changed to use RANGE instead of REF.
(main.group_min_max)
- ALL changed to REF
(main.key_diff)
- Key changed from ref + index_only to PRIMARY key for InnoDB, as
OPTIMIZER_ROW_LOOKUP_COST + OPTIMIZER_ROW_NEXT_FIND_COST is smaller than
OPTIMIZER_KEY_LOOKUP_COST + OPTIMIZER_KEY_NEXT_FIND_COST.
(main.join_outer_innodb)
- Cost changes printouts
(main.opt_trace*)
- Result order change
(innodb_gis.rtree)
The idea is that instead of marking all select_lex's with DISTINCT, we
only mark those that really need distinct result.
Benefits of this change:
- Temporary tables used with derived tables, UNION, IN are now smaller
as duplicates are removed already on the insert phase.
- The optimizer can now produce better plans with EQ_REF. This can be
seen from the tests where several queries does not anymore materialize
derived tables twice.
- Queries affected by 'in_predicate_conversion_threshold' where large IN
lists are converted to sub query produces better plans.
Other things:
- Removed on duplicate call to sel->init_select() in
LEX::add_primary_to_query_expression_body()
- I moved the testing of
tab->table->pos_in_table_list->is_materialized_derived()
in join_read_const_table() to the caller as it caused problems for
derived tables that could be proven to be const tables.
This also is likely to fix some bugs as if join_read_const_table()
was aborted, the table was left marked as JT_CONST, which cannot
be good. I added an ASSERT there for now that can be removed when
the code has been properly tested.
Deallocation of TABLE_LIST::dt_handler and TABLE_LIST::pushdown_derived
was performed in multiple places if code. This not only made the code
more difficult to maintain but also led to memory leaks and
ASAN heap-use-after-free errors.
This commit puts deallocation of TABLE_LIST::dt_handler and
TABLE_LIST::pushdown_derived to the single point - JOIN::cleanup()
FederatedX table may refer to a table with a different name on the
remote server:
test> CREATE TABLE t2 (...) ENGINE="FEDERATEDX"
CONNECTION="mysql://user:pass@192.168.1.111:9308/federatedx/t1";
test> select * from t2 where ...;
This could cause an issue with federated_pushdown=1, because FederatedX
pushes the query (or derived table's) text to the remote server. The remote
server will try to read from table t2 (while it should read from t1).
Solution: do not allow pushing down queries with tables that have different
db_name.table name on the local and remote server.
This patch also fixes:
MDEV-29863 Server crashes in federatedx_txn::acquire after select from the
FederatedX table with partitions
Solution: disallow pushdown when partitioned FederatedX tables are used.
MDEV-27036: repeated "table" key resolve for print_explain_json
MDEV-27036: duplicated keys in best_access_path
MDEV-27036: Explain_aggr_filesort::print_json_members: resolve duplicated "filesort" member in Json object
MDEV-27036: Explain_basic_join::
print_explain_json_interns fixed start_dups_weedout case for main.explain_json test
This bug happened when the HEAP temporary table used for the derived table
created for a derived handler of a remote engine of the federated type
became full and was converted to an Area table. For this conversion
the tmp_table_param parameter must be always taken from the select_unit
object created for the result of the derived table.
- select_describe() should not attempt to produce query plans
for subqueries if the query is handled by a Select Handler.
- JOIN::save_explain_data_intern should not add links to Explain_select
for children selects if:
1. The whole query is handled by the Select Handler, or
2. this select (and so its children) is handled by Derived Handler.
Backport to 10.4:
- Don't try to push down SELECTs that have a side effect
- In case the storage engine did support pushdown of SELECT with an INTO
clause, write the rows we've got from it into select->join->result,
and not thd->protocol. This way, SELECT ... INTO ... FROM
smart_engine_table will put the result into where instructed, and
NOT send it to the client.
- Don't try to push down SELECTs that have a side effect
- In case the storage engine did support pushdown of SELECT with an INTO
clause, write the rows we've got from it into select->join->result,
and not thd->protocol. This way, SELECT ... INTO ... FROM
smart_engine_table will put the result into where instructed, and
NOT send it to the client.
Count the "gap" time between table accesses and display it as
r_other_time_ms in the "table" element.
* The advantage of this approach is that it doesn't add any new
my_timer_cycles() calls.
* The disadvantage is that the definition of what is done during
"other time" is not that clear: it includes checking the WHERE
(for this table), constructing index lookup tuple (for the next table)
writing to GROUP BY temporary table (as we dont account for that time
separately [yet], etc)
MDEV-17631 select_handler for a full query pushdown
Interfaces + Proof of Concept for federatedx with test cases.
The interfaces have been developed for integration of ColumnStore engine.
