MariaDB is getting support for new-style query optimizer hints. The goals are control of the optimizer and compatibility with MySQL's hints. This talk covers some of technical details behind the hints implementation.

1. MariaDB's
New-Generation Optimizer Hints
Fine Optimizer Control and compatibility
Sergei Petrunia
MariaDB Foundation DeepDive
August 2015

2. Traditional ways to control the optimizer
●
Index Hints
– select * from tbl force index(idx1) where ...
●
Join Order Hints
– select straight_join col1 from t1, t2 ….
– select ... from t1 straight join t2 …
●
Session variables
– set optimizer_switch='materialization=off';
set join_cache_level=...;
set ...
select ...

3. New-style Hints
●
Special /*+ */ comment after main keyword
●
A lot of hints for access methods, optimizations, etc
– About 20 in total
– Covers what index and join order hints did
– Adds much more
●
Hints apply to query block, table, or index
select
/*+ JOIN_ORDER(t1 t2) INDEX(t1 idx1) */
col1, col2
from t1, t2
where ...

4. Origins of new-style Hints
●
Hints in Oracle Database
●
Introduced starting from MySQL 5.7
– Inherit the syntax and some concepts
– Hints themselves are different due to different optimizer
●
(what about the hypergraph optimizer? Do all hints apply?)
●
Hints in TiDB
select
/*+ JOIN_ORDER(t1 t2) INDEX(t1 idx1) */
...

5. MariaDB’s needs for hints
=> MariaDB is getting hints
Need fine
optimizer
control
Want
Compatibility
with MySQL
But not
”bug-for-bug”

6. MariaDB is getting New-style hints
●
MariaDB 12.0 (March 2025, rolling release, now stable)
– Initial support: Join Order, Subquery, some Access Method hints.
●
MariaDB 12.1 (June 2025, rolling release, now RC)
– More hints: Index use, control for most optimizations.
●
MariaDB 12.2 (no releases yet)
– Remaining optimizations: index_merge, Rowid Filter, etc.
●
MariaDB 12.3?
– ?

7. Hint walkthrough

8. Index hints
Similar to old-style:
(FORCE|IGNORE) INDEX [FOR (GROUP|ORDER|JOIN)}
select /*+ INDEX(tbl1 index1, index2, ...) */ ... from tbl1 ... ;
MySQL MariaDB
[NO_]INDEX Yes Yes
[NO_]GROUP_INDEX Yes Yes
[NO_]JOIN_INDEX Yes Yes
[NO_]ORDER_INDEX Yes Yes

9. Join order hints
MySQL MariaDB
JOIN_FIXED_ORDER Yes Yes
JOIN_ORDER Yes Yes
JOIN_PREFIX Yes Yes
JOIN_SUFFIX Yes Yes
select
/*+ JOIN_ORDER(TBL3, TBL2, t1) */ select_columns
from
t2 as TBL2
join t3 as TBL3 on ...
join t1 on ...
STRAIGHT_JOIN
…,T1,…,T2,…,T3,…
T1,T2,T3,……
……,T1,T2,T3

10. Easier to write than STRAIGHT JOIN
select
/*+ JOIN_ORDER(t1, TBL3, TBL2) */ select_columns
from
t2 as TBL2
join t3 as TBL3 on ...
join t1 on ...
●
“JOIN_ORDER(A,B,C)” is much easier than rewriting a query to
“A STRAIGHT JOIN B STRAIGHT JOIN C”
– ON expressions
– Outer joins (converted to inner)
– Etc
●
Hints use table aliases.

11. Gotcha: subquery merges
●
Semi-join WHERE IN (SELECT …) are processed in a similar way
●
Does merging make aliases ambiguous?
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
| 1 | SIMPLE | TBL1 | ALL | NULL | NULL | NULL | NULL | 10 | |
| 1 | SIMPLE | TBL1 | ALL | NULL | NULL | NULL | NULL | 1000 | Using join buffer (flat, BNL join) |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
| 1 | SIMPLE | TBL1 | ALL | NULL | NULL | NULL | NULL | 10 | |
| 1 | SIMPLE | TBL2 | ALL | NULL | NULL | NULL | NULL | 1000 | Using join buffer (flat, BNL join) |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
explain select * from t1 as TBL1, t2 as TBL2;
explain select * from t1 as TBL1, (select * from t2 as TBL1) as TBL2;

12. Ambiguous aliases
create view v1 as select * from t1 ... ;
select /*+ NO_INDEX(t1 a) */ * from v1,t1 where ...;
select /*+ JOIN_PREFIX(t1) */ * from v1,t1 where ... ;
●
This will apply to t1 outside v1:
●
This will apply to t1-inside-v1:
●
Hint “resolution” has different code paths
●
Causes counter-intuitive rules.
– Avoid this for now

13. Cost-based optimizations
MySQL MariaDB
[NO_]BKA Yes Yes
[NO_]BNL Yes Yes
[NO_]INDEX_MERGE Yes In progress
[NO_]MRR Yes Yes
[NO_]SKIP_SCAN Yes -
[NO_]ROWID_FILTER - In progress
[NO_]SPLIT_MATERIALIZED - Yes
●
NO_optname: disables use of the optimization
●
Optname: Try using(*) the optimization, even if it’s more expensive
(*)- or Encourage to use :-)

14. Subquery optimizations
●
Special hint syntax due to multiple strategies
●
Same meaning as cost-based.
MySQL MariaDB
SUBQUERY(strategy) Yes Yes
[NO_]SEMIJOIN(strategies) Yes Yes
/*+ [NO_]SEMIJOIN([query_block] sj_strategy,...) */
sj_strategy: FIRSTMATCH|LOOSESCAN|MATERIALIZATION|DUPSWEEDOUT
/*+ [NO_]SUBQUERY([query_block] strategy,...) */
strategy: MATERIALIZATION|INTOEXISTS

15. Does the Optimizer take hints?
●
Hints are “Best effort”, “Dumb interpretation”
TRAVEL /*+ USE_TRAM() */ FROM Helsinki TO Tallinn;
TRAVEL /*+ HAVE_BEER_BEFORE_ARRIVAL() */
FROM Helsinki TO Tallinn;

16. Non-cost-based optimizations
●
NO_optname: disable the optimization
●
optname: enable if disabled
– Not much point?
MySQL MariaDB
[NO_]MERGE Yes Yes
[NO_]DERIVED_CONDITION_PUSHDOWN Yes Yes
NO_ICP Yes Yes
NO_RANGE_OPTIMIZATION Yes Yes

17. Miscellaneous hints
MariaDB already has
●
●
Misc MySQL MariaDB
SET_VAR Yes Yes
MAX_EXECUTION_TIME Yes Yes
RESOURCE_GROUP Yes No
SET STATEMENT variable=value FOR ...;
SET STATEMENT max_statement_time=... FOR ...;
MariaDB doesn’t support resource groups.

18. Query Block hint
MySQL MariaDB
QB_NAME Yes Yes

19. Query blocks

20. Query block addressing
●
QB_NAME declares Query Block Name.
●
Hints control behavior in different query blocks.
with TOP_CUSTOMER as (
select /*+ QB_NAME(subq1) */ *
from customer C
order by C.balance desc limit 10
)
select /*+ INDEX(C@subq1 idx_balance) */ *
from TOP_CUSTOMER TC, orders ORD
where ...
with TOP_CUSTOMER as (
select /*+ QB_NAME(subq1) */ *
from customer C
order by C.balance desc limit 10
)
select /*+ INDEX(C@subq1 idx_balance) */ *
from TOP_CUSTOMER TC, orders ORD
where ...

21. Implicit Query Block names
● Implicit select#n name for every select
with TOP_CUSTOMER as (
select *
from customer C
order by C.balance desc limit 10
)
select /*+ INDEX(C@`select#2` idx_balance) */ *
from TOP_CUSTOMER TC, orders ORD
where ...

22. QB_NAME use cases
●
Does’t work in MySQL:
– Hints can’t control anything inside VIEWs
– QB_NAME has global visibilty
●
Conflicts?
●
CTEs that are used multiple times?
– Implicit names do not scale
select
/*+ hint1(arg1) hint2(arg2) hint(arg3, arg4) */ *
Col1,col2,somef_function_col(3(
from
Tbl1 join tbl2 on t1b.col1=tbl2.cond2 left join tbl3 ...
●
Can tune everything in the query from one location!

23. TiDB’s way – a possible solution?
●
QB_NAME assigns a name to Query Block elsewhere
●
Uses “path” through views
●
Uses SEL_n to find selects inside VIEWs.
●
=> We’re looking into this.
create view v1 as select ... from t where ...;
create view v2 as select ... from v1 where .... ;
select
/*+
QB_NAME(select1_in_v1, v2@SEL_1 .v1@SEL_1 .@SEL_1)
NO_ORDER_INDEX(t@select1_in_v1, idx1)
*/
col1
from v2;

24. From hints to saved query plans?

25. From Hints to Query Plans
●
Can we save the query plan as a set of hints?
●
In Oracle DB this was “stored outlines”
– Deprecated since Oracle 11g
●
TiDB doesn’t have it documented, but
TiDB> explain format='hint' select * from t10, t11 where t10.b=t11.a;
+--------------------------------------------------------------------------------------------------------+
| hint |
+--------------------------------------------------------------------------------------------------------+
| hash_join(@`sel_1` `test`.`t10`), use_index(@`sel_1` `test`.`t10` ), use_index(@`sel_1` `test`.`t11` ) |
+--------------------------------------------------------------------------------------------------------+
●
Is this worth doing?

26. Summary

27. Summary (1)
●
MariaDB is getting new-style hints.
– Introduced in MariaDB 12.0
– Reasonaby complete set in MariaDB 12.2
●
Hints are compatible with MySQL
– which were inspired by OracleDB’s
●
MariaDB hints cover
– MySQL’s hints, where applicable
– MariaDB’s optimizations

28. Summary (2)
●
Query block addressing can be improved
●
Coudl use hints for saving query plans in the
future?

29. Credits
●
Hints in MariaDB are brought to you by
– Oleg Smirnov
– Alexander Barkov
– Dave Gosselin
– Lena Startseva
– Sergei Petrunia

30. Thanks!