After this post I’ve got a question how one can tell if his outer join was converted to inner. You can find it out by looking at the warning generated by EXPLAIN EXTENDED. If the outer join wasn’t converted, you’ll see it in the rewritten query in the warning:
mysql> explain extended select * from t1 left join (t2, t3) on t2.a= t1.a; ... 3 rows in set, 1 warning (0.00 sec) mysql> show warnings\G *************************** 1. row *************************** Level: Note Code: 1003 Message: select `test`.`t1`.`a` AS `a`,`test`.`t2`.`a` AS `a`,`test`.`t3`.`a` AS `a` from `test`.`t1` left join (`test`.`t2` join `test`.`t3`) on ((`test`.`t2`. `a` = `test`.`t1`.`a`)) where 1
In this query LEFT JOIN is not converted to inner.
Now let’s try a query where outer join will be converted:
mysql> explain extended select * from t1 left join (t2, t3) on t2.a= t1.a where t2.a < 10; ... 3 rows in set, 1 warning (0.00 sec) mysql> show warnings\G *************************** 1. row *************************** Level: Note Code: 1003 Message: select `test`.`t1`.`a` AS `a`,`test`.`t2`.`a` AS `a`,`test`.`t3`.`a` AS `a` from `test`.`t1` join `test`.`t2` join `test`.`t3` where ((`test`.`t1`. `a` = `test`.`t2`.`a`) and (`test`.`t2`.`a` < 10))
Looking at the warning text we see an inner join instead of the left join, and also we see that the ON clause has been added into the WHERE.
Yes, those warning messages are hard to read, they have excessive quoting and the lines are too long. But at the moment certain kinds of query plan details are displayed only there, so skiming through the rewritten query text may pay off when you have doubts about what is going on.
Here is MySQL’s nested outer joins optimization cheat sheet:
- RIGHT JOIN is converted to LEFT JOIN. FULL JOIN is not supported.
- Outer joins are converted to inner joins when possible
Constraints on join order:
- “Outer tables go first”
- “No interleaving”
Table access rules:
- “Inner” table access methods are constructed from parts of the ON condition. WHERE condition can’t be used to construct table accesses.
- Parts of ON condition are checked as soon as possible
- Parts of the WHERE condition
- are not checked until we’ve found a row combination that matches the ON clause
- are checked as soon as possible after that.
Or, in more detail:
RIGHT JOIN to LEFT JOIN conversion is obvious:
(t1 RIGHT JOIN t2 ON cond) = (t2 LEFT JOIN t1 ON cond)
Conversion from outer to inner join is possible when the result of inner join will be the same. It will be the same if the row combination with NULL-complimented row will not pass the WHERE clause. For example, if we look at the query
t1 LEFT JOIN t2 ON some_cond WHERE t2.a=t1.b
we’ll see that a row with t2.a IS NULL will not satisfy the WHERE condition. Hence, this outer join can be converted to inner.
Constraints on join order
- Outer tables go first
- any outer table used in the outer join’s ON clause must be before all of the inner tables.
- No interleaving
- tables contained within an outer join must form a continuous sequence in the join order. Interleaving with tables that are outside of the outer join is not allowed.
Table access rules
Now, this requires some explanation. MySQL’s nested-loops join code tries to check parts of the WHERE as soon as
possible. For example when a query
SELECT * FROM t1,t2, ... WHERE t1.col1=c1 AND t2.col1=t1.col2 AND t2.col2=c3 AND ...
is executed using a join order of (t1, t2,…), it proceeds according to this kind of scenario:
We can see here that the the WHERE condition is split into parts that are checked “as early as possible”.
With outer joins is more complicated. We need to know if we’ll need to generate a NULL-complemented row combination. We won’t need to if there was a combination of inner tables that matched the ON (but not necessarily the WHERE) clause. The solution is to switch the WHERE parts checking on and off.
The best way to show it is with example: Suppose we have a query
SELECT * FROM ... ot1 LEFT JOIN (it1, it2) ON somecond WHERE ...
and suppose the join order is (ot1, it1, it2, …). The execution will proceed in this manner:
What’s visible there? When we start scanning table it1, we check only the ON condition. We can’t check the WHERE - we could iscard some it1’s row that is the only row that will match the ON condition, think there will be no matches, and erroneously generate the NULL-complimented row.
After we’ve found the match for the ON condition, we go back and check all parts of the WHERE we did not check because of the above mentioned reason.
After that, the execution proceeds as if this was an inner join, with ON merged into the WHERE clause.
The diagram also shows why we can’t use parts of the WHERE clause to create table acccess methods: because there are times when we can’t use parts of the WHERE for filtering. We always can use parts of the ON though.
Now it should be clear where all Table Access Rules came from.
I hope this is my last post about this topic. It seems we’ve resolved all of the issues and I’ll now describe the user-visible consequences.
To recall, we’re talking about subquery predicates in form
(oe1, ..., oeN) [NOT] IN (SELECT ie1, ..., ieN FROM ... )
that are located in a context where it matters if the predicate’s result is NULL or FALSE. The name “oe” stands for “outer expression”, ie stands for “inner expression”.
MySQL evaluates queries “from outside to inside”, i.e. we first get the values of (oe1, .. oeN) and then we run the subquery and capture the rows it produces. An apparent and very useful optimization is to “inform” the subquery that we’re looking only for rows that have “ie1=oe1″, “ie2=oe2″ and so on. This is done by injecting appropriate equalities into subquery’s WHERE (or HAVING) clause. That is,
(oe1, ..., oeN) [NOT] IN (SELECT ie1, ..., ieN FROM ... )
EXISTS (SELECT 1 /* ie1, ..., ieN */ FROM ... WHERE subquery_where AND oe1=ie1 AND oe2=ie2 AND ... oeN=ieN))
However, this conversion is only valid if we ignore possible NULL values. If some of the iek can be NULL, then we need to use oek=iek OR iek IS NULL instead. I’ve covered this case in detail here in NULL problem in the right part section.
Correct handling of cases where some oek IS NULL requires more radical changes. We’ve just made those changes and here they are:
The new strategy
According to SQL’s interpretation of NULL as “unknown value”,
NULL IN (non-empty-list-of-some-values) = NULL NULL IN () = FALSE
So, when we want to evaluate
NULL IN (SELECT ie FROM ... )
we need to run the SELECT and see if it will produce any rows. Note that we need to run the original SELECT, without any injected equalities mentioned in the previous section.
On the other hand, it is absolutely essential to have
not_null_oe IN (SELECT ie FROM ...)
EXISTS (SELECT 1 /* ie1 */ FROM ... WHERE ie1=not_null_oe ...)
If we don’t do this, subqueries will be terribly slow. We’ve solved this “inject or not inject” dilemma by wrapping the injected conditions into triggers. A subquery
(oe1, ..., oeN) [NOT] IN (SELECT ie1, ..., ieN FROM ... )
is converted into
EXISTS (SELECT 1 /* ie1, ..., ieN */ FROM ... WHERE subquery_where AND trigcond(oe1=ie1) AND trigcond(oeN=ieN) AND ... )
where each trigcond(X) is a special “magic” function defined as:
trigcond(X) := X when the "linked" outer expression oe_i is not NULL trigcond(X) := TRUE when the "linked" outer expression oe_i is NULL
Equalities that are wrapped into trigcond() function are not first class predicates for the query optimizer. Most optimizations cannot deal with predicates that may be turned on and off at query execution time, so they assume any trigcond(X) to be unknown function and ignore it. At the moment, triggerered equalities can be used by those optimizations:
- Ref-optimizer: trigcond(X=Y [OR Y IS NULL]) can be used to construct ref, eq_ref or ref_or_null table accesses.
- Index lookup-based subquery execution engines: trigcond(X=Y) can be used to construct unique_subquery or index_subquery access.
- Table condition generator: if the subquery is a join of several tables, triggered condition will be checked as soon as possible.
When the optimizer uses triggered condition to create some kind of index lookup-based access (#1 and #2 in the above list), it must have a strategy for the case when the condition is turned off. This “Plan B” strategy is always the same - do a full table scan. In EXPLAIN the plan B shows up as “Full scan on NULL key” in the “Extra” column:
mysql> explain select t1.col1, t1.col1 IN (select t2.key1 from t2 where t2.col2=t1.col2) from t1
*************************** 1. row *************************** id: 1 select_type: PRIMARY table: t1 ... *************************** 2. row *************************** id: 2 select_type: DEPENDENT SUBQUERY table: t2 type: index_subquery possible_keys: key1 key: key1 key_len: 5 ref: func rows: 2 Extra: Using where; Full scan on NULL key
And if you run EXPLAIN EXTENDED …; SHOW WARNINGS you can see the triggered condition:
*************************** 2. row *************************** Level: Note Code: 1003 Message: select `test`.`t1`.`col1` AS `col1`,<in_optimizer>(`test`.`t1`.`col1`,<exists>(< index_lookup>(<cache>(`test`.`t1`.`col1`) in t2 on key1 checking NULL where (`test`.`t2`. `col2` = `test`.`t1`.`col2`) having trigcond(<is_not_null_test>(`test`.`t2`.`key1`))))) AS `t1.col1 IN (select t2.key1 from t2 where t2.col2=t1.col2)` from `test`.`t1`
The first apparent implication is that NULL IN (SELECT …) now may cause full table scans (slow!) where it previously did not. This is the price to pay for correct results.
For multi-table subqueries the execution of NULL IN (SELECT …) is going to be particularly slow because the join optimizer doesn’t optimize for the case when outer expression is NULL. It assumes that subquery evaluations with NULL on the left side are very rare, even if there is statistics that says otherwise
On the other hand, if you have left expression that may be NULL but actually never is, you will not lose any speed.
The practical hints are
- A column must be declared as NOT NULL if it really is. This is important for the other parts of the query optimizer too.
- If you don’t really need the correct NULL/FALSE answer, you can easily avoid the slow execution path: just replace
oe IN (SELECT ie FROM ...)
(oe IS NOT NULL) AND (oe IN (SELECT ie FROM ...))
and NULL IN (SELECT …) will never be evaluated because MySQL stops evaluating AND parts as soon as the answer is clear.
The goal of this new strategy was to improve compliance and not speed. However we’ve had an intent to not make anything unneccessarily slow. If something became slower for you please file a bug, perhaps we’ll be able to do something about it.
A while ago I wrote about problem with NULLs and IN/=ANY subqueries MySQL had. I was completely correct when I wrote that the fix won’t be simple. It took 3 bug entries (BUG#8804, BUG#24085, BUG#24127), several review iterations by Igor, and the final patch is around 2,300 lines long.
The good news is that this patch solves the problem completely, and it is already in the 5.0.36 tree. The documentation is not yet updated, doing that is now on my todo. There is quite a lot to document: we’ve had to introduce “switchable access methods”, where a table accessed using ref is sometimes accessed using full table scan. (for the impatient: no, the new access method is not not called ref_or_null_or_all , it is still ref[_or_null] but with “Full scan on NULL key” in the “Extra” column).
I’ll post here when the docs become available.