MariaDB server crashes when a query includes a derived table
containing unnamed column (eg: `SELECT '' from t`). When `Item`
object representing such unnamed column was checked for valid,
non-empty name in `TABLE_LIST::create_field_translation`, the
server crahsed(assertion `item->name.str && item->name.str[0]`
failed).
This fix removes the redundant assertion. The assert was a strict
debug guard that's no longer needed because the code safely handles
empty strings without it.
Selecting `''` from a derived table caused `item->name.str`
to be an empty string. While the pointer itself wasn't `NULL`
(`item->name.str` is `true`), its first character (`item->name.str[0]`)
was null terminator, which evaluates to `false` and eventually made
the assert fail. The code immediately after the assert can safely
handle empty strings and the assert was guarding against something
which the code can already handle.
Includes `mysql-test/main/derived.test` to verify the fix.
(Variant 3) (commit in 11.4)
When a derived table has a GROUP BY clause:
SELECT ...
FROM (SELECT ... GROUP BY col1, col2) AS tbl
The optimizer would use inner join's output cardinality as an estimate
of derived table size, ignoring the fact that GROUP BY operation would
produce much fewer groups.
Add code to produce tighter bounds:
- The GROUP BY list is split into per-table lists. If GROUP BY list has
expressions that refer to multiple tables, we fall back to join output
cardinality.
- For each table, the first cardinality estimate is join_tab->read_records.
- Then, we try to get a tighter bound by using index statistics.
- If indexes do not cover all GROUP BY columns, we try to use per-column
EITS statistics.
When a derived table which has distinct values and BLOB fields is
materialized, an index is created over all columns to ensure only
unique values are placed to the result.
This index is created in a special mode HA_UNIQUE_HASH to support BLOBs.
Later the optimizer may incorrectly choose this index to retrieve values
from the derived table, although such type of index cannot be used
for data retrieval.
This commit excludes HA_UNIQUE_HASH indexes from adding to
`JOIN::keyuse` array thus preventing their subsequent usage for
data retrieval
The code in create_internal_tmp_table() didn't take into account that
now temporary (derived) tables may have multiple indexes:
- one index due to duplicate removal
= In this example created by conversion of big-IN(...) into subquery
= this index might be converted into a "unique constraint" if the key
length is too large.
- one index added by derived_with_keys optimization.
Make create_internal_tmp_table() handle multiple indexes.
Before this patch, use of a unique constraint was indicated in
TABLE_SHARE::uniques. This was ok as unique constraint was the only index
in the table. Now it's no longer the case so TABLE_SHARE::uniques is removed
and replaced with an in-memory-only flag HA_UNIQUE_HASH.
This patch is based on Monty's patch.
Co-Author: Monty <monty@mariadb.org>
This patch fixes not only the assertion failure in the function
Field_iterator_table_ref::set_field_iterator() but also:
- fixes the problem of forced materialization of derived tables used
in subqueries contained in WHERE clauses of single-table and multi-table
UPDATE and DELETE statements
- fixes the problem of MDEV-17954 that prevented execution of multi-table
DELETE statements if they use in their WHERE clauses references to
the tables that are updated.
The patch must be considered a complement to the patch for MDEV-28883.
Approved by Oleksandr Byelkin <sanja@mariadb.com>
Firstmatch_picker::check_qep() has an optimization that allows firstmatch
to be used together with join buffer under some conditions. In this
case the cost was assumed to be same as what best_access_path()
had calculated.
However if HASH+join_buffer was used, then
fix_semijoin_strategies_for_picked_join_order() would remove the
join_buffer (which would cause a full join to be used) and the cost
assumption by Firstmatch_picker::check_qep() would be wrong.
Later check_join_cache_usage() sees that it's a full scan and decides
it can use join buffering, (But not the hash join).
Fixed by also allowing HASH joins with firstmatch.
This removes the need to change disable and re-enable join buffer.
Test case changes:
- HASH join used with firstmatch (Using join buffer (flat, BNLH join))
- Filtered could change with firstmatch as the conversion with and without
join_buffered lost the filtering information.
- The not "re-enabling join buffer" is shown in main.optimizer_trace
Original code by Sergei, optimized by Monty.
Author: Sergei Petrunia <sergey@mariadb.com>, monty@mariadb.org
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 bug was some old code that, without any explanation, reset
PART_KEY_FLAG from fields in temporary tables. This caused
join_tab->key_dependent to not be updated properly, which caused
an assert.
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)
This includes all test changes from
"Changing all cost calculation to be given in milliseconds"
and forwards.
Some of the things that caused changes in the result files:
- As part of fixing tests, I added 'echo' to some comments to be able to
easier find out where things where wrong.
- MATERIALIZED has now a higher cost compared to X than before. Because
of this some MATERIALIZED types have changed to DEPENDEND SUBQUERY.
- Some test cases that required MATERIALIZED to repeat a bug was
changed by adding more rows to force MATERIALIZED to happen.
- 'Filtered' in SHOW EXPLAIN has in many case changed from 100.00 to
something smaller. This is because now filtered also takes into
account the smallest possible ref access and filters, even if they
where not used. Another reason for 'Filtered' being smaller is that
we now also take into account implicit filtering done for subqueries
using FIRSTMATCH.
(main.subselect_no_exists_to_in)
This is caluculated in best_access_path() and stored in records_out.
- Table orders has changed because more accurate costs.
- 'index' and 'ALL' for small tables has changed to use 'range' or
'ref' because of optimizer_scan_setup_cost.
- index can be changed to 'range' as 'range' optimizer assumes we don't
have to read the blocks from disk that range optimizer has already read.
This can be confusing in the case where there is no obvious where clause
but instead there is a hidden 'key_column > NULL' added by the optimizer.
(main.subselect_no_exists_to_in)
- Scan on primary clustered key does not report 'Using Index' anymore
(It's a table scan, not an index scan).
- For derived tables, the number of rows is now 100 instead of 2,
which can be seen in EXPLAIN.
- More tests have "Using index for group by" as the cost of this
optimization is now more correct (lower).
- A primary key could be preferred for a normal key, even if it would
access more rows, as it's faster to do 1 lokoup and 3 'index_next' on a
clustered primary key than one lookup trough a secondary.
(main.stat_tables_innodb)
Notes:
- There was a 4.7% more calls to best_extension_by_limited_search() in
the main.greedy_optimizer test. However examining the test results
it looked that the plans where slightly better (eq_ref where more
chained together) so I assume this is ok.
- I have verified a few test cases where there was notable/unexpected
changes in the plan and in all cases the new optimizer plans where
faster. (main.greedy_optimizer and some others)
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.
Before this patch, when calculating the cost of fetching and using a
row/key from the engine, we took into account the cost of finding a
row or key from the engine, but did not consistently take into account
index only accessed, clustered key or covered keys for all access
paths.
The cost of the WHERE clause (TIME_FOR_COMPARE) was not consistently
considered in best_access_path(). TIME_FOR_COMPARE was used in
calculation in other places, like greedy_search(), but was in some
cases (like scans) done an a different number of rows than was
accessed.
The cost calculation of row and index scans didn't take into account
the number of rows that where accessed, only the number of accepted
rows.
When using a filter, the cost of index_only_reads and cost of
accessing and disregarding 'filtered rows' where not taken into
account, which made filters cost less than there actually where.
To remedy the above, the following key & row fetch related costs
has been added:
- The cost of fetching and using a row is now split into different costs:
- key + Row fetch cost (as before) but multiplied with the variable
'optimizer_cache_cost' (default to 0.5). This allows the user to
tell the optimizer the likehood of finding the key and row in the
engine cache.
- ROW_COPY_COST, The cost copying a row from the engine to the
sql layer or creating a row from the join_cache to the record
buffer. Mostly affects table scan costs.
- ROW_LOOKUP_COST, the cost of fetching a row by rowid.
- KEY_COPY_COST the cost of finding the next key and copying it from
the engine to the SQL layer. This is used when we calculate the cost
index only reads. It makes index scans more expensive than before if
they cover a lot of rows. (main.index_merge_myisam)
- KEY_LOOKUP_COST, the cost of finding the first key in a range.
This replaces the old define IDX_LOOKUP_COST, but with a higher cost.
- KEY_NEXT_FIND_COST, the cost of finding the next key (and rowid).
when doing a index scan and comparing the rowid to the filter.
Before this cost was assumed to be 0.
All of the above constants/variables are now tuned to be somewhat in
proportion of executing complexity to each other. There is tuning
need for these in the future, but that can wait until the above are
made user variables as that will make tuning much easier.
To make the usage of the above easy, there are new (not virtual)
cost calclation functions in handler:
- ha_read_time(), like read_time(), but take optimizer_cache_cost into
account.
- ha_read_and_copy_time(), like ha_read_time() but take into account
ROW_COPY_TIME
- ha_read_and_compare_time(), like ha_read_and_copy_time() but take
TIME_FOR_COMPARE into account.
- ha_rnd_pos_time(). Read row with row id, taking ROW_COPY_COST
into account. This is used with filesort where we don't need
to execute the WHERE clause again.
- ha_keyread_time(), like keyread_time() but take
optimizer_cache_cost into account.
- ha_keyread_and_copy_time(), like ha_keyread_time(), but add
KEY_COPY_COST.
- ha_key_scan_time(), like key_scan_time() but take
optimizer_cache_cost nto account.
- ha_key_scan_and_compare_time(), like ha_key_scan_time(), but add
KEY_COPY_COST & TIME_FOR_COMPARE.
I also added some setup costs for doing different types of scans and
creating temporary tables (on disk and in memory). This encourages
the optimizer to not use these for simple 'a few row' lookups if
there are adequate key lookup strategies.
- TABLE_SCAN_SETUP_COST, cost of starting a table scan.
- INDEX_SCAN_SETUP_COST, cost of starting an index scan.
- HEAP_TEMPTABLE_CREATE_COST, cost of creating in memory
temporary table.
- DISK_TEMPTABLE_CREATE_COST, cost of creating an on disk temporary
table.
When calculating cost of fetching ranges, we had a cost of
IDX_LOOKUP_COST (0.125) for doing a key div for a new range. This is
now replaced with 'io_cost * KEY_LOOKUP_COST (1.0) *
optimizer_cache_cost', which matches the cost we use for 'ref' and
other key lookups. The effect is that the cost is now a bit higher
when we have many ranges for a key.
Allmost all calculation with TIME_FOR_COMPARE is now done in
best_access_path(). 'JOIN::read_time' now includes the full
cost for finding the rows in the table.
In the result files, many of the changes are now again close to what
they where before the "Update cost for hash and cached joins" commit,
as that commit didn't fix the filter cost (too complex to do
everything in one commit).
The above changes showed a lot of a lot of inconsistencies in
optimizer cost calculation. The main objective with the other changes
was to do calculation as similar (and accurate) as possible and to make
different plans more comparable.
Detailed list of changes:
- Calculate index_only_cost consistently and correctly for all scan
and ref accesses. The row fetch_cost and index_only_cost now
takes into account clustered keys, covered keys and index
only accesses.
- cost_for_index_read now returns both full cost and index_only_cost
- Fixed cost calculation of get_sweep_read_cost() to match other
similar costs. This is bases on the assumption that data is more
often stored on SSD than a hard disk.
- Replaced constant 2.0 with new define TABLE_SCAN_SETUP_COST.
- Some scan cost estimates did not take into account
TIME_FOR_COMPARE. Now all scan costs takes this into
account. (main.show_explain)
- Added session variable optimizer_cache_hit_ratio (default 50%). By
adjusting this on can reduce or increase the cost of index or direct
record lookups. The effect of the default is that key lookups is now
a bit cheaper than before. See usage of 'optimizer_cache_cost' in
handler.h.
- JOIN_TAB::scan_time() did not take into account index only scans,
which produced a wrong cost when index scan was used. Changed
JOIN_TAB:::scan_time() to take into consideration clustered and
covered keys. The values are now cached and we only have to call
this function once. Other calls are changed to use the cached
values. Function renamed to JOIN_TAB::estimate_scan_time().
- Fixed that most index cost calculations are done the same way and
more close to 'range' calculations. The cost is now lower than
before for small data sets and higher for large data sets as we take
into account how many keys are read (main.opt_trace_selectivity,
main.limit_rows_examined).
- Ensured that index_scan_cost() ==
range(scan_of_all_rows_in_table_using_one_range) +
MULTI_RANGE_READ_INFO_CONST. One effect of this is that if there
is choice of doing a full index scan and a range-index scan over
almost the whole table then index scan will be preferred (no
range-read setup cost). (innodb.innodb, main.show_explain,
main.range)
- Fixed the EQ_REF and REF takes into account clustered and covered
keys. This changes some plans to use covered or clustered indexes
as these are much cheaper. (main.subselect_mat_cost,
main.state_tables_innodb, main.limit_rows_examined)
- Rowid filter setup cost and filter compare cost now takes into
account fetching and checking the rowid (KEY_NEXT_FIND_COST).
(main.partition_pruning heap.heap_btree main.log_state)
- Added KEY_NEXT_FIND_COST to
Range_rowid_filter_cost_info::lookup_cost to account of the time
to find and check the next key value against the container
- Introduced ha_keyread_time(rows) that takes into account finding
the next row and copying the key value to 'record'
(KEY_COPY_COST).
- Introduced ha_key_scan_time() for calculating an index scan over
all rows.
- Added IDX_LOOKUP_COST to keyread_time() as a startup cost.
- Added index_only_fetch_cost() as a convenience function to
OPT_RANGE.
- keyread_time() cost is slightly reduced to prefer shorter keys.
(main.index_merge_myisam)
- All of the above caused some index_merge combinations to be
rejected because of cost (main.index_intersect). In some cases
'ref' where replaced with index_merge because of the low
cost calculation of get_sweep_read_cost().
- Some index usage moved from PRIMARY to a covering index.
(main.subselect_innodb)
- Changed cost calculation of filter to take KEY_LOOKUP_COST and
TIME_FOR_COMPARE into account. See sql_select.cc::apply_filter().
filter parameters and costs are now written to optimizer_trace.
- Don't use matchings_records_in_range() to try to estimate the number
of filtered rows for ranges. The reason is that we want to ensure
that 'range' is calculated similar to 'ref'. There is also more work
needed to calculate the selectivity when using ranges and ranges and
filtering. This causes filtering column in EXPLAIN EXTENDED to be
100.00 for some cases where range cannot use filtering.
(main.rowid_filter)
- Introduced ha_scan_time() that takes into account the CPU cost of
finding the next row and copying the row from the engine to
'record'. This causes costs of table scan to slightly increase and
some test to changed their plan from ALL to RANGE or ALL to ref.
(innodb.innodb_mysql, main.select_pkeycache)
In a few cases where scan time of very small tables have lower cost
than a ref or range, things changed from ref/range to ALL.
(main.myisam, main.func_group, main.limit_rows_examined,
main.subselect2)
- Introduced ha_scan_and_compare_time() which is like ha_scan_time()
but also adds the cost of the where clause (TIME_FOR_COMPARE).
- Added small cost for creating temporary table for
materialization. This causes some very small tables to use scan
instead of materialization.
- Added checking of the WHERE clause (TIME_FOR_COMPARE) of the
accepted rows to ROR costs in get_best_ror_intersect()
- Removed '- 0.001' from 'join->best_read' and optimize_straight_join()
to ensure that the 'Last_query_cost' status variable contains the
same value as the one that was calculated by the optimizer.
- Take avg_io_cost() into account in handler::keyread_time() and
handler::read_time(). This should have no effect as it's 1.0 by
default, except for heap that overrides these functions.
- Some 'ref_or_null' accesses changed to 'range' because of cost
adjustments (main.order_by)
- Added scan type "scan_with_join_cache" for optimizer_trace. This is
just to show in the trace what kind of scan was used.
- When using 'scan_with_join_cache' take into account number of
preceding tables (as have to restore all fields for all previous
table combination when checking the where clause)
The new cost added is:
(row_combinations * ROW_COPY_COST * number_of_cached_tables).
This increases the cost of join buffering in proportion of the
number of tables in the join buffer. One effect is that full scans
are now done earlier as the cost is then smaller.
(main.join_outer_innodb, main.greedy_optimizer)
- Removed the usage of 'worst_seeks' in cost_for_index_read as it
caused wrong plans to be created; It prefered JT_EQ_REF even if it
would be much more expensive than a full table scan. A related
issue was that worst_seeks only applied to full lookup, not to
clustered or index only lookups, which is not consistent. This
caused some plans to use index scan instead of eq_ref (main.union)
- Changed federated block size from 4096 to 1500, which is the
typical size of an IO packet.
- Added costs for reading rows to Federated. Needed as there is no
caching of rows in the federated engine.
- Added ha_innobase::rnd_pos_time() cost function.
- A lot of extra things added to optimizer trace
- More costs, especially for materialization and index_merge.
- Make lables more uniform
- Fixed a lot of minor bugs
- Added 'trace_started()' around a lot of trace blocks.
- When calculating ORDER BY with LIMIT cost for using an index
the cost did not take into account the number of row retrivals
that has to be done or the cost of comparing the rows with the
WHERE clause. The cost calculated would be just a fraction of
the real cost. Now we calculate the cost as we do for ranges
and 'ref'.
- 'Using index for group-by' is used a bit more than before as
now take into account the WHERE clause cost when comparing
with 'ref' and prefer the method with fewer row combinations.
(main.group_min_max).
Bugs fixed:
- Fixed that we don't calculate TIME_FOR_COMPARE twice for some plans,
like in optimize_straight_join() and greedy_search()
- Fixed bug in save_explain_data where we could test for the wrong
index when displaying 'Using index'. This caused some old plans to
show 'Using index'. (main.subselect_innodb, main.subselect2)
- Fixed bug in get_best_ror_intersect() where 'min_cost' was not
updated, and the cost we compared with was not the one that was
used.
- Fixed very wrong cost calculation for priority queues in
check_if_pq_applicable(). (main.order_by now correctly uses priority
queue)
- When calculating cost of EQ_REF or REF, we added the cost of
comparing the WHERE clause with the found rows, not all row
combinations. This made ref and eq_ref to be regarded way to cheap
compared to other access methods.
- FORCE INDEX cost calculation didn't take into account clustered or
covered indexes.
- JT_EQ_REF cost was estimated as avg_io_cost(), which is half the
cost of a JT_REF key. This may be true for InnoDB primary key, but
not for other unique keys or other engines. Now we use handler
function to calculate the cost, which allows us to handle
consistently clustered, covered keys and not covered keys.
- ha_start_keyread() didn't call extra_opt() if keyread was already
enabled but still changed the 'keyread' variable (which is wrong).
Fixed by not doing anything if keyread is already enabled.
- multi_range_read_info_cost() didn't take into account io_cost when
calculating the cost of ranges.
- fix_semijoin_strategies_for_picked_join_order() used the wrong
record_count when calling best_access_path() for SJ_OPT_FIRST_MATCH
and SJ_OPT_LOOSE_SCAN.
- Hash joins didn't provide correct best_cost to the upper level, which
means that the cost for hash_joins more expensive than calculated
in best_access_path (a difference of 10x * TIME_OF_COMPARE).
This is fixed in the new code thanks to that we now include
TIME_OF_COMPARE cost in 'read_time'.
Other things:
- Added some 'if (thd->trace_started())' to speed up code
- Removed not used function Cost_estimate::is_zero()
- Simplified testing of HA_POS_ERROR in get_best_ror_intersect().
(No cost changes)
- Moved ha_start_keyread() from join_read_const_table() to join_read_const()
to enable keyread for all types of JT_CONST tables.
- Made a few very short functions inline in handler.h
Notes:
- In main.rowid_filter the join order of order and lineitem is swapped.
This is because the cost of doing a range fetch of lineitem(98 rows) is
almost as big as the whole join of order,lineitem. The filtering will
also ensure that we only have to do very small key fetches of the rows
in lineitem.
- main.index_merge_myisam had a few changes where we are now using
less keys for index_merge. This is because index scans are now more
expensive than before.
- handler->optimizer_cache_cost is updated in ha_external_lock().
This ensures that it is up to date per statements.
Not an optimal solution (for locked tables), but should be ok for now.
- 'DELETE FROM t1 WHERE t1.a > 0 ORDER BY t1.a' does not take cost of
filesort into consideration when table scan is chosen.
(main.myisam_explain_non_select_all)
- perfschema.table_aggregate_global_* has changed because an update
on a table with 1 row will now use table scan instead of key lookup.
TODO in upcomming commits:
- Fix selectivity calculation for ranges with and without filtering and
when there is a ref access but scan is chosen.
For this we have to store the lowest known value for
'accepted_records' in the OPT_RANGE structure.
- Change that records_read does not include filtered rows.
- test_if_cheaper_ordering() needs to be updated to properly calculate
costs. This will fix tests like main.order_by_innodb,
main.single_delete_update
- Extend get_range_limit_read_cost() to take into considering
cost_for_index_read() if there where no quick keys. This will reduce
the computed cost for ORDER BY with LIMIT in some cases.
(main.innodb_ext_key)
- Fix that we take into account selectivity when counting the number
of rows we have to read when considering using a index table scan to
resolve ORDER BY.
- Add new calculation for rnd_pos_time() where we take into account the
benefit of reading multiple rows from the same page.
This bug manifested itself when the server processed a query containing
a derived table over union whose ORDER BY clause included a subquery
with unresolvable column reference. For such a query the server crashed
when trying to resolve column references in the ORDER BY clause used by
union.
For any union with ORDER BY clause an extra SELECT_LEX structure is created
and it is attached to SELECT_LEX_UNIT structure of the union via the field
fake_select_lex. The outer context for fake_select_lex must be the same as
for other selects of the union. If the union is used in the FROM list of
a derived table then the outer context for fake_select_lex must be set to
NULL in line with other selects of the union. It was not done and it
caused a crash when searching for possible resolution of an unresolvable
column reference occurred in a subquery used in the ORDER BY clause.
Approved by Oleksandr Byelkin <sanja@mariadb.com>
In main.index_merge_myisam we remove the test that was added in
commit a2d24def8c because
it duplicates the test case that was added in
commit 5af12e4635.
- multi_range_read_info_const now uses the new records_in_range interface
- Added handler::avg_io_cost()
- Don't calculate avg_io_cost() in get_sweep_read_cost if avg_io_cost is
not 1.0. In this case we trust the avg_io_cost() from the handler.
- Changed test_quick_select to use TIME_FOR_COMPARE instead of
TIME_FOR_COMPARE_IDX to align this with the rest of the code.
- Fixed bug when using test_if_cheaper_ordering where we didn't use
keyread if index was changed
- Fixed a bug where we didn't use index only read when using order-by-index
- Added keyread_time() to HEAP.
The default keyread_time() was optimized for blocks and not suitable for
HEAP. The effect was the HEAP prefered table scans over ranges for btree
indexes.
- Fixed get_sweep_read_cost() for HEAP tables
- Ensure that range and ref have same cost for simple ranges
Added a small cost (MULTI_RANGE_READ_SETUP_COST) to ranges to ensure
we favior ref for range for simple queries.
- Fixed that matching_candidates_in_table() uses same number of records
as the rest of the optimizer
- Added avg_io_cost() to JT_EQ_REF cost. This helps calculate the cost for
HEAP and temporary tables better. A few tests changed because of this.
- heap::read_time() and heap::keyread_time() adjusted to not add +1.
This was to ensure that handler::keyread_time() doesn't give
higher cost for heap tables than for normal tables. One effect of
this is that heap and derived tables stored in heap will prefer
key access as this is now regarded as cheap.
- Changed cost for index read in sql_select.cc to match
multi_range_read_info_const(). All index cost calculation is now
done trough one function.
- 'ref' will now use quick_cost for keys if it exists. This is done
so that for '=' ranges, 'ref' is prefered over 'range'.
- scan_time() now takes avg_io_costs() into account
- get_delayed_table_estimates() uses block_size and avg_io_cost()
- Removed default argument to test_if_order_by_key(); simplifies code
If a derived table has SELECT DISTINCT, provide index statistics for it so that the join optimizer in the
upper select knows that ref access to the table will produce one row.
The logic and the implementation scheme are similar with the
MDEV-9197 Pushdown conditions into non-mergeable views/derived tables
How the push down is made on the example:
select * from t1
where a>3 and b>10 and
(a,b) in (select x,max(y) from t2 group by x);
-->
select * from t1
where a>3 and b>10 and
(a,b) in (select x,max(y)
from t2
where x>3
group by x
having max(y)>10);
The implementation scheme:
1. Search for the condition cond that depends only on the fields
from the left part of the IN subquery (left_part)
2. Find fields F_group in the select of the right part of the
IN subquery (right_part) that are used in the GROUP BY
3. Extract from the cond condition cond_where that depends only on the
fields from the left_part that stay at the same places in the left_part
(have the same indexes) as the F_group fields in the projection of the
right_part
4. Transform cond_where so it can be pushed into the WHERE clause of the
right_part and delete cond_where from the cond
5. Transform cond so it can be pushed into the HAVING clause of the right_part
The optimization is made in the
Item_in_subselect::pushdown_cond_for_in_subquery() and is controlled by the
variable condition_pushdown_for_subquery.
New test file in_subq_cond_pushdown.test is created.
There are also some changes made for setup_jtbm_semi_joins().
Now it is decomposed into the 2 procedures: setup_degenerate_jtbm_semi_joins()
that is called before optimize_cond() for cond and setup_jtbm_semi_joins()
that is called after optimize_cond().
New setup_jtbm_semi_joins() is made in the way so that the result of its work is
the same as if it was called before optimize_cond().
The code that is common for pushdown into materialized derived and into materialized
IN subqueries is factored out into pushdown_cond_for_derived(),
Item_in_subselect::pushdown_cond_for_in_subquery() and
st_select_lex::pushdown_cond_into_where_clause().
