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Added chapter about subselect transformations
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bell@sanja.is.com.ua
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1 changed files with 237 additions and 2 deletions
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@ -54,6 +54,7 @@ This is a manual about @strong{MySQL} internals.
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* flush tables:: How MySQL Handles @code{FLUSH TABLES}
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* filesort:: How MySQL Does Sorting (@code{filesort})
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* selects:: How MySQL performs different selects
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* transformations:: How MySQL transforms subqueries
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* coding guidelines:: Coding Guidelines
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* mysys functions:: Functions In The @code{mysys} Library
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* DBUG:: DBUG Tags To Use
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@ -261,7 +262,7 @@ and then we read the rows in the sorted order into a row buffer
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@end itemize
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@node selects, coding guidelines, flush tables, Top
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@node selects, transformations, flush tables, Top
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@chapter How MySQL performs different selects
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@node select steps,,,
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@ -604,7 +605,241 @@ mysql_explain_union for every inner UNION
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PROBLEM: how it will work with global query optimization?
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@node coding guidelines, mysys functions, selects, Top
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@node transformations, coding guidelines, selects, Top
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@chapter How MySQL transforms subqueries
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Item_subselect virtual method select_transformer is used to rewrite
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subqueries. It is called from Item_subselect::init (which called in
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Item_subselect constructor)
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@node transformation IN
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@section Item_in_subselect::select_transformer
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Item_in_subselect::select_transformer is divided on two parts for
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scalar left part and row left part:
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@node transformation scalar IN
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@subsection Scalar IN Subselect
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To rewrite scalar IN subselect used method
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Item_in_subselect::single_value_transformer, Scalar IN subselect will
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be replaced with Item_in_optimizer.
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Item_in_optimizer item is special boolean function. On value request
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(one of val, val_int or val_str methods) it evaluate left expression of
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IN by storing it value in cache item (one of Item_cache* items), then it
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test cache is it NULL. If left expression (cache) is NULL then
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Item_in_optimizer return NULL, else it evaluate Item_in_subselect.
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Example queries.
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@example
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a) SELECT * from t1 where t1.a in (SELECT t2.a FROM t2);
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b) SELECT * from t1 where t1.a in (SELECT t2.a FROM t2 GROUP BY t2.a);
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@end example
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@itemize
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@item
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Item_in_subselect inherit mechanism of getting value from
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Item_exists_subselect.
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@item
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Select_transformer stores reference to left expression in its
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conditions: (in WHERE in case 'a' and in a HAVING in case 'b')
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@item
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Item from item list of this select (t2.a) can be referred with special
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reference (Item_ref_null_helper or Item_asterisk_remover).
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This reference informs Item_in_optimizer if item (t2.a) is NULL by
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setting the 'was_null' flag.
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@item
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The return value from Item_in_subselect will be evaluated as following:
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@itemize @bullet
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@item
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If TRUE return true
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@item
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If NULL return null
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@item
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If FALSE and 'was_null' is set, return null
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@item
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return FALSE
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@end itemize
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@end itemize
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<left_expression> IN (SELECT <item> ...) will be represented like
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following:
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@example
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+-----------------+
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|Item_in_optimizer|
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+-----------------+
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+---------------------+------------+
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| |
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+-----------------------+ +-----------------+
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| <left_expression> | |Item_in_subselect|
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| | +-----------------+
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+-----------------------+ |
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|<left_expression cache>| +-----------+-----------+
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| | | |
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+-----------------------+ | |
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^ +----------+ +--------------------+
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+<<<<<<<<<<<<<<<<<| Item_ref | +<<<|Item_ref_null_helper|
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+----------+ V +--------------------+
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V +--------------------+
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+>>>| <item> |
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+--------------------+
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@end example
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where '<<<<<<<<<' is reference in meaning of Item_ref.
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Item_ref used for point to <left_expression cache>, because in time of
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transformation we know only address of variable where pointer on cache
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will be stored.
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If select have ORDER BY clause it will be wiped out, because no sense in
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ORDER BY without LIMIT here.
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If IN subselect union condition of every select in UNION will be changed
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personally.
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Following is examples of IN transformations:
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@example
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a) <left_expression> IN (SELECT <item> FROM t
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WHERE <where_exp>)
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will be represented as
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(SELECT 1 FROM t
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WHERE <where_exp> and
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Item_ref(<cached_left_expression>)=<Item_asterisk_remover(<Item>)>)
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b) <left_expression> IN (SELECT <item> FROM t
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HAVING <having_expr>
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ORDER BY 1)
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will be represented as
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(SELECT <item> as ref_null_helper FROM t
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HAVING <having_exp> AND
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Item_ref(<cached_left_expression>) = ref_null_helper)
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c) <left_expression> IN (SELECT <item> UNION ...)
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will be represented as
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(SELECT 1
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HAVING Item_ref(<cached_left_expression>)=
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<Item_asterisk_remover(<Item>)>
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UNION ...)
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(having without FROM is syntax error, but having condition is checked
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even for subselect without FROM)
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d) <left_expression> IN (select <item>)
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will be completely replaced with <left_expression> = <item>
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@end example
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Now conditions (WHERE (a) or HAVING (b)) will be changed depends of
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select in following way:
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If subselect have HAVING , sum function or GROUP BY (case a) then item
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list will be unchanged and Item_ref_null_helper reference will be
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created on item list element. Condition will be added to HAVING condition.
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If subselect have not HAVING, sum function or GROUP BY (case b) then:
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@itemize @bullet
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@item
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@strong{item list} will be replaced with 1.
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@item
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@strong{<item>} from item list will be stored in Item_asterisk_remover, which
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inherit from Item_ref_null_helper, but store item on which refer by
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itself, and also it can resolve '*' item.
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@item
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@strong{<left_expression cache> = <Item_ref_null_helper>} will be added to
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WHERE clause this item or to HAVING clause if this subselect have
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no FROM clause and subselect is union (case c).
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@end itemize
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Single select without FROM will be reduced to just
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<left_expression> = <item> without using Item_in_optimizer.
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@node transformations row IN
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@subsection Row IN Subselect
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To rewrite row IN subselect used method
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Item_in_subselect::row_value_transformer. It work in almost same way as
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scalar analog, but work with Item_cache_row for caching left expression
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and use references on elements of Item_cache_row.
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To refer on item list it use Item_ref_on_list_position.
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Item_ref_on_list_position::fix_fields will find item in item list of
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subselect by number and create Item_ref_null_helper to refer on it. It
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used to find reference when all '*' items will be translated in item
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list. Subselect with have HAVING, sum functions or GROUP BY will
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transformed in following way:
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@example
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ROW(l1, l2, ... lN) IN (SELECT i1, i2, ... iM FROM t HAVING <having_expr>)
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will be following:
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(SELECT i1, i2, ... iM FROM t
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HAVING <having_expr> and
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<cache_l1> = <ref_on_list_position(1)> AND
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<cache_l2> = <ref_on_list_position(2)> AND
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...
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<cache_lN> = <ref_on_list_position(N)>)
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@end example
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In this way will be transformed select without FROM, too.
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For other subselect it will be same but for WHERE clause.
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@node transformations all any
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@section Item_allany_subselect
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Item_allany_subselect is inherited from Item_in_subselect.
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ALL/ANY/SOME use same algorithm (and same method of Item_in_subselect)
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as scalar IN, but use different function instead of '='.
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ANY/SOME use same function that was listed after left expression.
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ALL use inverted function, and all subselect passed as argument to
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Item_func_not.
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@node transformations singlerow
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@section Item_singlerow_subselect
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Item_singlerow_subselect will be rewritten only if it have not FROM
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clause, it is not part of UNION and it is scalar subselect. For now will
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not be converted subselects with field or reference on top of item list
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(we can't change name of such items from one hand, but from other hand
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we should assign to it name of whole subselect which will be reduced);
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Following will not be reduced:
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@example
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SELECT a;
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SELECT 1 UNION SELECT 2;
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SELECT 1 FROM t1;
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@end example
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Following select will be reduced:
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@example
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SELECT 1;
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SELECT a+2;
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@end example
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Such subselect will be completely replaced by its expression from item
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list and its SELECT_LEX and SELECT_LEX_UNIT will be removed from
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SELECT_LEX's tree.
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But all Item_fields and Item_ref of that expression will be marked for
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special fix_fields() procedure. fix_fields() for such Item will be
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performed is same way as for items of inner subselect. Also if this
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expression is Item_fields or Item_ref then name of this new item will
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be same as name of this item (but not '(SELECT ...)'). It is done to
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prevent broke references on such items from more inner subselects.
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@node coding guidelines, mysys functions, transformations, Top
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@chapter Coding Guidelines
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@itemize @bullet
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