The code in choose_best_splitting() assumed that the join prefix is
in join->positions[].
This is not necessarily the case. This function might be called when
the join prefix is in join->best_positions[], too.
Follow the approach from best_access_path(), which calls this function:
pass the current join prefix as an argument,
"const POSITION *join_positions" and use that.
When a query does implicit grouping and join operation produces an empty
result set, a NULL-complemented row combination is generated.
However, constant table fields still show non-NULL values.
What happens in the is that end_send_group() is called with a
const row but without any rows matching the WHERE clause.
This last part is shown by 'join->first_record' not being set.
This causes item->no_rows_in_result() to be called for all items to reset
all sum functions to their initial state. However fields are not set
to NULL.
The used fix is to produce NULL-complemented records for constant tables
as well. Also, reset the constant table's records back in case we're
in a subquery which may get re-executed.
An alternative fix would have item->no_rows_in_result() also work
with Item_field objects.
There is some other issues with the code:
- join->no_rows_in_result_called is used but never set.
- Tables that are used with group functions are not properly marked as
maybe_null, which is required if the table rows should be regarded as
null-complemented (not existing).
- The code that tries to detect if mixed_implicit_grouping should be set
didn't take into account all usage of fields and sum functions.
- Item_func::restore_to_before_no_rows_in_result() called the wrong
function.
- join->clear() does not use a table_map argument to clear_tables(),
which caused it to ignore constant tables.
- unclear_tables() does not correctly restore status to what is
was before clear_tables().
Main bug fix was to always use a table_map argument to clear_tables() and
always use join->clear() and clear_tables() together with unclear_tables().
Other fixes:
- Fixed Item_func::restore_to_before_no_rows_in_result()
- Set 'join->no_rows_in_result_called' when no_rows_in_result_set()
is called.
- Removed not used argument from setup_end_select_func().
- More code comments
- Ensure that end_send_group() modifies the same fields as are in the
result set.
- Changed return_zero_rows() to use pointers instead of references,
similar to the rest of the code.
Reviewer: Sergei Petrunia <sergey@mariadb.com>
The problem was trying to access JOIN_TAB::select which is set to NULL
when using the filesort. The correct way is accessing either
JOIN_TAB::select or JOIN_TAB::filesort->select depending on whether
the filesort is used.
This commit introduces member function JOIN_TAB::get_sql_select()
encapsulating that check so the code duplication is eliminated.
The new condition (s->table->quick_keys.is_set(best_key->key))
was added to best_access_path() to eliminate a Valgrind error.
The cause of that error was using TRASH_ALLOC(quick_key_parts)
instead of bzero(quick_key_parts); hence, accessing
s->table->quick_key_parts[best_key->key]) without prior checking
for quick_keys.is_set() might have caused reading "dirty" memory
The problem was that join_buffer_size conflicted with
join_buffer_space_limit, which caused the query to be run without join
buffer. However this caused wrong results as the optimizer assumed
that hash+join buffer would ensure that the equi-join condition
would be satisfied, and didn't check it itself.
Fixed by not using join_buffer_space_limit when
optimize_join_buffer_size=off. This matches the documentation at
https://mariadb.com/kb/en/block-based-join-algorithms
Other things:
- Removed not used variable JOIN_TAB::join_buffer_size_limit
- Give an error if we cannot allocate a join buffer. This can
only happen if the join_buffer variables are wrongly configured or
we are running out of memory.
In the future, instead of returning an error, we could properly
convert the query plan that uses BNL-H join into one that doesn't
use join buffering:
make sure the equi-join condition is checked where appropriate.
Reviewer: Sergei Petrunia <sergey@mariadb.com>
This patch optimizes the number of refills for the lateral derived table
to which a materialized derived table subject to split optimization is
is converted. This optimized number of refills is now considered as the
expected number of refills of the materialized derived table when searching
for the best possible splitting of the table.
When a query does implicit grouping and join operation produces an empty
result set, a NULL-complemented row combination is generated.
However, constant table fields still show non-NULL values.
What happens in the is that end_send_group() is called with a
const row but without any rows matching the WHERE clause.
This last part is shown by 'join->first_record' not being set.
This causes item->no_rows_in_result() to be called for all items to reset
all sum functions to their initial state. However fields are not set
to NULL.
The used fix is to produce NULL-complemented records for constant tables
as well. Also, reset the constant table's records back in case we're
in a subquery which may get re-executed.
An alternative fix would have item->no_rows_in_result() also work
with Item_field objects.
There is some other issues with the code:
- join->no_rows_in_result_called is used but never set.
- Tables that are used with group functions are not properly marked as
maybe_null, which is required if the table rows should be regarded as
null-complemented (not existing).
- The code that tries to detect if mixed_implicit_grouping should be set
didn't take into account all usage of fields and sum functions.
- Item_func::restore_to_before_no_rows_in_result() called the wrong
function.
- join->clear() does not use a table_map argument to clear_tables(),
which caused it to ignore constant tables.
- unclear_tables() does not correctly restore status to what is
was before clear_tables().
Main bug fix was to always use a table_map argument to clear_tables() and
always use join->clear() and clear_tables() together with unclear_tables().
Other fixes:
- Fixed Item_func::restore_to_before_no_rows_in_result()
- Set 'join->no_rows_in_result_called' when no_rows_in_result_set()
is called.
- Removed not used argument from setup_end_select_func().
- More code comments
- Ensure that end_send_group() modifies the same fields as are in the
result set.
- Changed return_zero_rows() to use pointers instead of references,
similar to the rest of the code.
Rewrite datetime comparison conditions into sargeable. For example,
YEAR(col) <= val -> col <= YEAR_END(val)
YEAR(col) < val -> col < YEAR_START(val)
YEAR(col) >= val -> col >= YEAR_START(val)
YEAR(col) > val -> col > YEAR_END(val)
YEAR(col) = val -> col BETWEEN YEAR_START(val) AND YEAR_END(val)
Do the same with DATE(col), for example:
DATE(col) <= val -> col <= DAY_END(val)
After such a rewrite index lookup on column "col" can be employed
In block-nl-join, add:
- r_loops - this shows how many incoming record combinations this
query plan node had.
- r_effective_rows - this shows the average number of matching rows
that this table had for each incoming record combination. This is
comparable with r_rows in non-blocked access methods.
For BNL-joins, it is always equal to
$.table.r_rows * $.table.r_filtered
For BNL-H joins the value cannot be computed from other values
Reviewed by: Monty <monty@mariadb.org>
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
This error was discovered while working on
MDEV-30540 Wrong result with IN list length reaching
IN_PREDICATE_CONVERSION_THRESHOLD
If there is read error from handler::ha_rnd_next() during a recursive
query, st_select_lex_unit::exec_recursive() will crash as it will try to
get the error code from a structure that was deleted by the callee.
The code was using the construct:
sl->join->exec();
saved_error=sl->join->error;
This does not work as sl->join was freed by the exec() and sl->join would
be set to 0.
Fixed by having JOIN::exec() return the error code.
The included test case simulates the error in ha_rnd_next(), which causes
a crash without the patch.
scovered whle working on
MDEV-30540 Wrong result with IN list length reaching
IN_PREDICATE_CONVERSION_THRESHOLD
If there is read error from handler::ha_rnd_next() during a recursive
query, st_select_lex_unit::exec_recursive() will crash as it will try to
get the error code from a structure that was deleted by the callee.
The code was using the construct:
sl->join->exec();
saved_error=sl->join->error;
This does not work as sl->join was freed by the exec() and sl->join was
set to 0.
Fixed by having JOIN::exec() return the error code.
The included test case simulates the error in ha_rnd_next(), which causes
a crash without the patch.
The old code in prev_record_reads() did give wrong estimates when a
join_buffer was used or if the table was depending on more than one
other tables. When join_cache is used, it will cause a re-order of row
combinations, which causes more calls to the engine for tables that
are depending on tables before the join_cached one.
The new prev_records_read() code provides more exact estimates and
should never give a 'too low estimate', assuming that the data to the
function is correct
The definition of prev_record_read() is also updated.
The new definition is:
"Estimate the number of engine ha_index_read_calls for EQ_REF tables
when taking into account the one-row-cache in join_read_always_key()"
The cost of using prev_record_reads() value is changed. The value is
now used similar as before to calculate the cost of the storage engine
calls. However the cost of the WHERE cost is changed to take into
account the total number of row combinations as the WHERE has to be
checked even if the one-row-cache is used. This makes the cost
slightly higher than before (for the same prev_record_reads() value).
Other things:
- Cached return value of prev_record_read() in best_access_path() to
avoid some function calls.
- Fixed bug where position[].use_join_buffer was set in
best_acess_path() when join buffer was not used. This confused the
semi join optimizer to try to reoptimize plans that did not need to be
reoptimized.
The effect of the bug fix is that we avoid doing some re-optimziations
with semi-joins when join_buffer is not used. In these cases the value
shown for the 'Filtering' column in EXPLAIN EXTENDED may change.
- Added 'prev_record.cc' that was used to verify the logic in
prev_record_reads().
Changes in test suite:
- EQ_REF tables are moved up to be earlier. This is because either the
higher WHERE cost when EQ_REF is used with more row combination or
change of cost when using join_cache.
- Filtered has changed (to the better) for some cases using semi-joins
subselect_sj.test subselect_sj_jcl6.test
Amended patch from Monty:
The issue was that Loose_scan_opt::save_to_position() did not take
into account records_out from best_access_path()
Make sure that POSITION object filled by Loose_scan_opt::save_to_position()
has records_out not higher than any other possible access method.
Detailed description:
- Added more function comments and fixed types in some old comments
- Removed an outdated comment
- Cleaned up some functions in records.cc
- Replaced "while" with "if"
- Reused error code
- Made functions similar
- Added caching of pfs_batch_update()
- Simplified some rowid_filter code
- Only call build_range_rowid_filter() if rowid filter will be used
- Replaced tab->is_rowid_filter_built with need_to_build_rowid_filter.
We only have to test need_to_build_rowid_filter to know if we have
to build the filter. Old code needed two tests
- Added function 'clear_range_rowid_filter' to disable rowid filter.
Made things simpler as we can now clear all rowid filter variables
in one place.
- Removed some 'if' in sub_select()
This patch is the result of running
run-clang-tidy -fix -header-filter=.* -checks='-*,modernize-use-equals-default' .
Code style changes have been done on top. The result of this change
leads to the following improvements:
1. Binary size reduction.
* For a -DBUILD_CONFIG=mysql_release build, the binary size is reduced by
~400kb.
* A raw -DCMAKE_BUILD_TYPE=Release reduces the binary size by ~1.4kb.
2. Compiler can better understand the intent of the code, thus it leads
to more optimization possibilities. Additionally it enabled detecting
unused variables that had an empty default constructor but not marked
so explicitly.
Particular change required following this patch in sql/opt_range.cc
result_keys, an unused template class Bitmap now correctly issues
unused variable warnings.
Setting Bitmap template class constructor to default allows the compiler
to identify that there are no side-effects when instantiating the class.
Previously the compiler could not issue the warning as it assumed Bitmap
class (being a template) would not be performing a NO-OP for its default
constructor. This prevented the "unused variable warning".
- table_after_join_selectivity() should use records_init (new bug)
- get_examined_rows() changed to double to get similar results
as in MariaDB 10.11
- Fixed bug where table_after_join_selectivity() did not correct
selectivity in the case where a RANGE is used instead of a REF.
This can happen if the range can use more key_parts than the REF.
WHERE key_part1=10 and key_part2 < 10
Other things:
- Use JT_RANGE instead of JT_ALL for RANGE access in all parts of the code.
Before we used JT_ALL for RANGE.
- Force RANGE be used in best_access_path() if the range used more key
parts than ref. In the original code, this was done much later in
make_join_select)(). However we need to know in
table_after_join_selectivity() if we have used RANGE or not.
- Added more information about filtering to optimizer_trace.
This makes it easier to compare different costs and also allows
the optimizer to optimizer different storage engines more reliably.
- Added tests/check_costs.pl, a tool to verify optimizer cost calculations.
- Most engine costs has been found with this program. All steps to
calculate the new costs are documented in Docs/optimizer_costs.txt
- User optimizer_cost variables are given in microseconds (as individual
costs can be very small). Internally they are stored in ms.
- Changed DISK_READ_COST (was DISK_SEEK_BASE_COST) from a hard disk cost
(9 ms) to common SSD cost (400MB/sec).
- Removed cost calculations for hard disks (rotation etc).
- Changed the following handler functions to return IO_AND_CPU_COST.
This makes it easy to apply different cost modifiers in ha_..time()
functions for io and cpu costs.
- scan_time()
- rnd_pos_time() & rnd_pos_call_time()
- keyread_time()
- Enhanched keyread_time() to calculate the full cost of reading of a set
of keys with a given number of ranges and optional number of blocks that
need to be accessed.
- Removed read_time() as keyread_time() + rnd_pos_time() can do the same
thing and more.
- Tuned cost for: heap, myisam, Aria, InnoDB, archive and MyRocks.
Used heap table costs for json_table. The rest are using default engine
costs.
- Added the following new optimizer variables:
- optimizer_disk_read_ratio
- optimizer_disk_read_cost
- optimizer_key_lookup_cost
- optimizer_row_lookup_cost
- optimizer_row_next_find_cost
- optimizer_scan_cost
- Moved all engine specific cost to OPTIMIZER_COSTS structure.
- Changed costs to use 'records_out' instead of 'records_read' when
recalculating costs.
- Split optimizer_costs.h to optimizer_costs.h and optimizer_defaults.h.
This allows one to change costs without having to compile a lot of
files.
- Updated costs for filter lookup.
- Use a better cost estimate in best_extension_by_limited_search()
for the sorting cost.
- Fixed previous issues with 'filtered' explain column as we are now
using 'records_out' (min rows seen for table) to calculate filtering.
This greatly simplifies the filtering code in
JOIN_TAB::save_explain_data().
This change caused a lot of queries to be optimized differently than
before, which exposed different issues in the optimizer that needs to
be fixed. These fixes are in the following commits. To not have to
change the same test case over and over again, the changes in the test
cases are done in a single commit after all the critical change sets
are done.
InnoDB changes:
- Updated InnoDB to not divide big range cost with 2.
- Added cost for InnoDB (innobase_update_optimizer_costs()).
- Don't mark clustered primary key with HA_KEYREAD_ONLY. This will
prevent that the optimizer is trying to use index-only scans on
the clustered key.
- Disabled ha_innobase::scan_time() and ha_innobase::read_time() and
ha_innobase::rnd_pos_time() as the default engine cost functions now
works good for InnoDB.
Other things:
- Added --show-query-costs (\Q) option to mysql.cc to show the query
cost after each query (good when working with query costs).
- Extended my_getopt with GET_ADJUSTED_VALUE which allows one to adjust
the value that user is given. This is used to change cost from
microseconds (user input) to milliseconds (what the server is
internally using).
- Added include/my_tracker.h ; Useful include file to quickly test
costs of a function.
- Use handler::set_table() in all places instead of 'table= arg'.
- Added SHOW_OPTIMIZER_COSTS to sys variables. These are input and
shown in microseconds for the user but stored as milliseconds.
This is to make the numbers easier to read for the user (less
pre-zeros). Implemented in 'Sys_var_optimizer_cost' class.
- In test_quick_select() do not use index scans if 'no_keyread' is set
for the table. This is what we do in other places of the server.
- Added THD parameter to Unique::get_use_cost() and
check_index_intersect_extension() and similar functions to be able
to provide costs to called functions.
- Changed 'records' to 'rows' in optimizer_trace.
- Write more information to optimizer_trace.
- Added INDEX_BLOCK_FILL_FACTOR_MUL (4) and INDEX_BLOCK_FILL_FACTOR_DIV (3)
to calculate usage space of keys in b-trees. (Before we used numeric
constants).
- Removed code that assumed that b-trees has similar costs as binary
trees. Replaced with engine calls that returns the cost.
- Added Bitmap::find_first_bit()
- Added timings to join_cache for ANALYZE table (patch by Sergei Petrunia).
- Added records_init and records_after_filter to POSITION to remember
more of what best_access_patch() calculates.
- table_after_join_selectivity() changed to recalculate 'records_out'
based on the new fields from best_access_patch()
Bug fixes:
- Some queries did not update last_query_cost (was 0). Fixed by moving
setting thd->...last_query_cost in JOIN::optimize().
- Write '0' as number of rows for const tables with a matching row.
Some internals:
- Engine cost are stored in OPTIMIZER_COSTS structure. When a
handlerton is created, we also created a new cost variable for the
handlerton. We also create a new variable if the user changes a
optimizer cost for a not yet loaded handlerton either with command
line arguments or with SET
@@global.engine.optimizer_cost_variable=xx.
- There are 3 global OPTIMIZER_COSTS variables:
default_optimizer_costs The default costs + changes from the
command line without an engine specifier.
heap_optimizer_costs Heap table costs, used for temporary tables
tmp_table_optimizer_costs The cost for the default on disk internal
temporary table (MyISAM or Aria)
- The engine cost for a table is stored in table_share. To speed up
accesses the handler has a pointer to this. The cost is copied
to the table on first access. If one wants to change the cost one
must first update the global engine cost and then do a FLUSH TABLES.
This was done to be able to access the costs for an open table
without any locks.
- When a handlerton is created, the cost are updated the following way:
See sql/keycaches.cc for details:
- Use 'default_optimizer_costs' as a base
- Call hton->update_optimizer_costs() to override with the engines
default costs.
- Override the costs that the user has specified for the engine.
- One handler open, copy the engine cost from handlerton to TABLE_SHARE.
- Call handler::update_optimizer_costs() to allow the engine to update
cost for this particular table.
- There are two costs stored in THD. These are copied to the handler
when the table is used in a query:
- optimizer_where_cost
- optimizer_scan_setup_cost
- Simply code in best_access_path() by storing all cost result in a
structure. (Idea/Suggestion by Igor)
Added code to support that force index can be used to force an index scan
instead of a full table scan. Currently this code is disable but I added
a test to verify that things works if the code is ever enabled.
Other things:
- FORCE INDEX will now work with "Range checked for each record" and
join cache (see main/type_time_6065)
- Removed code ifdef with BAD_OPTIMIZATION (New cost calculations should
fix this).
- Removed TABLE_LIST->force_index and comment that it should be removed
- Added TABLE->force_index_join and use in the corresponding places.
This means that FORCE INDEX FOR ORDER BY will not affect keys used
in joins anymore.
Remove TODO that the above should be added.
I still kept TABLE->force_index as it's used in
test_if_cheaper_ordering() and opt_range.cc
- Removed setting table->force_index when calling test_quick_select() as
it's not needed (force_index is an argument to test_quick_select())
One effect of this change in the test suite is that tests with very few
rows changed to use sub queries instead of materialization. This is
correct and expected as for these the materialization overhead is too high.
A lot of tests where fixed to still use materialization by adding a
few rows to the tables (most tests has only 2-3 rows and are thus easily
affected when cost computations are changed).
Other things:
- Added more variables to TMPTABLE_COSTS for better cost calculation
- Added cost of copying rows to TMPTABLE_COSTS lookup and write
- Added THD::optimizer_cache_hit_ratio for easier cost calculations
- Added DISK_FAST_READ_SIZE to be used when calculating costs when
reading big blocks from a disk
This cleans up the interface for choose_plan() as it is not depending
on setting join->emb_sj_nest.
choose_plan() now sets up join->emb_sj_nest and join->allowed_tables before
calling optimize_straight_join() and best_extension_by_limited_search().
Other things:
- Converted some 'if' to DBUG_ASSERT() as these should always be true.
- Calculate 'allowed_tables' in choose_plan() as this never changes in
the childs.
- Added assert to check that next_emb->nested_join->n_tables doesn't
get to a wrong value.
- Documented some variables in sql_select.h
