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mariadb/sql/opt_table_elimination.cc
Sergey Petrunya c1b69e72eb MWL#17: Table elimination 
- Switch from trying to eliminate all tables at once (which didn't work)
  to the original approach of bottom-up elimination.
2009-08-18 16:03:58 +03:00

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/**
@file
@brief
Table Elimination Module
@defgroup Table_Elimination Table Elimination Module
@{
*/
#ifdef USE_PRAGMA_IMPLEMENTATION
#pragma implementation // gcc: Class implementation
#endif
#include "mysql_priv.h"
#include "my_bit.h"
#include "sql_select.h"
/*
OVERVIEW
This file contains table elimination module. The idea behind table
elimination is as follows: suppose we have a left join
SELECT * FROM t1 LEFT JOIN
(t2 JOIN t3) ON t3.primary_key=t1.col AND
t4.primary_key=t2.col
such that
* columns of the inner tables are not used anywhere ouside the outer join
(not in WHERE, not in GROUP/ORDER BY clause, not in select list etc etc),
* inner side of the outer join is guaranteed to produce at most one matching
record combination for each record combination of outer tables.
then the inner side of the outer join can be removed from the query, as it
will always produce only one record combination (either real or
null-complemented one) and we don't care about what that record combination
is.
MODULE INTERFACE
The module has one entry point - eliminate_tables() function, which one
needs to call (once) at some point before the join optimization.
eliminate_tables() operates over the JOIN structures. Logically, it
removes the right sides of outer join nests. Physically, it changes the
following members:
* Eliminated tables are marked as constant and moved to the front of the
join order.
* In addition to this, they are recorded in JOIN::eliminated_tables bitmap.
* Items that became disused because they were in the ON expression of an
eliminated outer join are notified by means of the Item tree walk which
calls Item::mark_as_eliminated_processor for every item
- At the moment the only Item that cares whether it was eliminated is
Item_subselect with its Item_subselect::eliminated flag which is used
by EXPLAIN code to check if the subquery should be shown in EXPLAIN.
Table elimination is redone on every PS re-execution.
TABLE ELIMINATION ALGORITHM
As said above, we can remove inner side of an outer join if it is
1. not referred to from any other parts of the query
2. always produces one matching record combination.
We check #1 by doing a recursive descent down the join->join_list while
maintaining a union of used_tables() attribute of all expressions we've seen
"elsewhere". When we encounter an outer join, we check if the bitmap of
tables on its inner side has an intersection with tables that are used
elsewhere. No intersection means that inner side of the outer join could
potentially be eliminated.
In order to check #2, one needs to prove that inner side of an outer join
is functionally dependent on the outside. We prove dependency by proving
functional dependency of intermediate objects:
- Inner side of outer join is functionally dependent when each of its tables
are functionally dependent. (We assume a table is functionally dependent
when its dependencies allow to uniquely identify one table record, or no
records).
- Table is functionally dependent when it has got a unique key whose columns
are functionally dependent.
- A column is functionally dependent when we could locate an AND-part of a
certain ON clause in form
tblX.columnY= expr
where expr is functionally-depdendent.
Apparently the above rules can be applied recursively. Also, certain entities
depend on multiple other entities. We model this by a bipartite graph which
has two kinds of nodes:
Value nodes:
- Table column values (each is a value of tblX.columnY)
- Table nodes (each node represents a table inside an eliminable join nest).
each value is either bound (i.e. functionally dependent) or not.
Module nodes:
- Nodes representing tblX.colY=expr equalities. Equality node has
= incoming edges from columns used in expr
= outgoing edge to tblX.colY column.
- Nodes representing unique keys. Unique key has
= incoming edges from key component value nodes
= outgoing edge to key's table node
- Inner side of outer join node. Outer join node has
= incoming edges from table value nodes
= No outgoing edges. Once we reach it, we know we can eliminate the
outer join.
A module may depend on multiple values, and hence its primary attribute is
the number of its depedencies that are not bound.
The algorithm starts with equality nodes that don't have any incoming edges
(their expressions are either constant or depend only on tables that are
outside of any outer joins) and proceeds to traverse dependency->dependant
edges until we've other traversed everything (TODO rephrase elaborate), or
we've reached the point where all outer join modules have zero unsatisfied
dependencies.
*/
class Value_dep;
class Field_value;
class Table_value;
class Module_dep;
class Equality_module;
class Outer_join_module;
class Key_module;
class Table_elimination;
/*
A value, something that can be bound or not bound. Also, values can be linked
in a list.
*/
class Value_dep : public Sql_alloc
{
public:
enum {
VALUE_FIELD,
VALUE_TABLE,
} type; /* Type of the object */
Value_dep(): bound(FALSE), next(NULL)
{}
bool bound;
Value_dep *next;
};
/*
A table field value. There is exactly only one such object for any tblX.fieldY
- the field epends on its table and equalities
- expressions that use the field are its dependencies
*/
class Field_value : public Value_dep
{
public:
Field_value(Table_value *table_arg, Field *field_arg) :
table(table_arg), field(field_arg)
{
type= Value_dep::VALUE_FIELD;
}
Table_value *table; /* Table this field is from */
Field *field;
/*
Field_deps that belong to one table form a linked list. list members are
ordered by field_index
*/
Field_value *next_table_field;
uint bitmap_offset; /* Offset of our part of the bitmap */
};
/*
A table value. There is one Table_value object for every table that can
potentially be eliminated.
- table depends on any of its unique keys
- has its fields and embedding outer join as dependency.
*/
class Table_value : public Value_dep
{
public:
Table_value(TABLE *table_arg) :
table(table_arg), fields(NULL), keys(NULL), outer_join_dep(NULL)
{
type= Value_dep::VALUE_TABLE;
}
TABLE *table;
Field_value *fields; /* Ordered list of fields that belong to this table */
Key_module *keys; /* Ordered list of Unique keys in this table */
Outer_join_module *outer_join_dep; /* Innermost eliminable outer join we're in */
};
/*
A 'module'. Module has dependencies
*/
class Module_dep : public Sql_alloc
{
public:
enum {
MODULE_EXPRESSION,
MODULE_MULTI_EQUALITY,
MODULE_UNIQUE_KEY,
MODULE_OUTER_JOIN
} type; /* Type of the object */
/*
Used to make a linked list of elements that became bound and thus can
make elements that depend on them bound, too.
*/
Module_dep *next;
uint unknown_args;
Module_dep() : next(NULL), unknown_args(0) {}
};
/*
A "tbl.column= expr" equality dependency. tbl.column depends on fields
used in expr.
*/
class Equality_module : public Module_dep
{
public:
Field_value *field;
Item *expression;
/* Used during condition analysis only, similar to KEYUSE::level */
uint level;
};
/*
A Unique key.
- Unique key depends on all of its components
- Key's table is its dependency
*/
class Key_module: public Module_dep
{
public:
Key_module(Table_value *table_arg, uint keyno_arg, uint n_parts_arg) :
table(table_arg), keyno(keyno_arg), next_table_key(NULL)
{
type= Module_dep::MODULE_UNIQUE_KEY;
unknown_args= n_parts_arg;
}
Table_value *table; /* Table this key is from */
uint keyno;
/* Unique keys form a linked list, ordered by keyno */
Key_module *next_table_key;
};
/*
An outer join nest that is subject to elimination
- it depends on all tables inside it
- has its parent outer join as dependency
*/
class Outer_join_module: public Module_dep
{
public:
Outer_join_module(//TABLE_LIST *table_list_arg,
uint n_children)
// table_list(table_list_arg), parent(NULL)
{
type= Module_dep::MODULE_OUTER_JOIN;
unknown_args= n_children;
}
/*
Outer join we're representing. This can be a join nest or one table that
is outer join'ed.
*/
// TABLE_LIST *table_list;
/* Parent eliminable outer join, if any */
// Outer_join_module *parent;
};
/*
Table elimination context
*/
class Table_elimination
{
public:
Table_elimination(JOIN *join_arg) : join(join_arg), n_outer_joins(0)
{
bzero(table_deps, sizeof(table_deps));
}
JOIN *join;
/* Array of equality dependencies */
Equality_module *equality_deps;
uint n_equality_deps; /* Number of elements in the array */
/* tablenr -> Table_value* mapping. */
Table_value *table_deps[MAX_KEY];
/* Outer joins that are candidates for elimination */
List<Outer_join_module> oj_deps;
uint n_outer_joins;
/* Bitmap of how expressions depend on bits */
MY_BITMAP expr_deps;
};
static
bool build_eq_deps_for_cond(Table_elimination *te, Equality_module **fdeps,
uint *and_level, Item *cond,
table_map usable_tables);
static
bool add_eq_dep(Table_elimination *te, Equality_module **eq_dep,
uint and_level,
Item_func *cond, Item *left, Item *right,
table_map usable_tables);
static
Equality_module *merge_func_deps(Equality_module *start, Equality_module *new_fields,
Equality_module *end, uint and_level);
static Table_value *get_table_value(Table_elimination *te, TABLE *table);
static Field_value *get_field_value(Table_elimination *te, Field *field);
static Outer_join_module *get_outer_join_dep(Table_elimination *te,
//TABLE_LIST *outer_join,
table_map deps_map);
static
bool run_elimination_wave(Table_elimination *te, Module_dep *bound_modules);
void eliminate_tables(JOIN *join);
static void mark_as_eliminated(JOIN *join, TABLE_LIST *tbl);
#ifndef DBUG_OFF
static void dbug_print_deps(Table_elimination *te);
#endif
/*******************************************************************************************/
/*
Produce Eq_dep elements for given condition.
SYNOPSIS
build_eq_deps_for_cond()
te Table elimination context
fdeps INOUT Put produced equality conditions here
and_level INOUT AND-level (like in add_key_fields)
cond Condition to process
usable_tables Tables which fields we're interested in. That is,
Equality_module represent "tbl.col=expr" and we'll
produce them only if tbl is in usable_tables.
DESCRIPTION
This function is modeled after add_key_fields()
*/
static
bool build_eq_deps_for_cond(Table_elimination *te, Equality_module **fdeps,
uint *and_level, Item *cond,
table_map usable_tables)
{
if (cond->type() == Item_func::COND_ITEM)
{
List_iterator_fast<Item> li(*((Item_cond*) cond)->argument_list());
Equality_module *org_key_fields= *fdeps;
/* AND/OR */
if (((Item_cond*) cond)->functype() == Item_func::COND_AND_FUNC)
{
Item *item;
while ((item=li++))
{
if (build_eq_deps_for_cond(te, fdeps, and_level, item, usable_tables))
return TRUE;
}
/*
TODO: inject here a "if we have {t.col=const AND t.col=smth_else}, then
remove the second part" logic.
*/
for (; org_key_fields != *fdeps ; org_key_fields++)
org_key_fields->level= *and_level;
}
else
{
(*and_level)++;
if (build_eq_deps_for_cond(te, fdeps, and_level, li++, usable_tables))
return TRUE;
Item *item;
while ((item=li++))
{
Equality_module *start_key_fields= *fdeps;
(*and_level)++;
if (build_eq_deps_for_cond(te, fdeps, and_level, item, usable_tables))
return TRUE;
*fdeps= merge_func_deps(org_key_fields, start_key_fields, *fdeps,
++(*and_level));
}
}
return FALSE;
}
if (cond->type() != Item::FUNC_ITEM)
return FALSE;
Item_func *cond_func= (Item_func*) cond;
Item **args= cond_func->arguments();
switch (cond_func->functype()) {
case Item_func::IN_FUNC:
{
if (cond_func->argument_count() == 2)
{
if (add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1],
usable_tables) ||
add_eq_dep(te, fdeps, *and_level, cond_func, args[1], args[0],
usable_tables))
return TRUE;
}
}
case Item_func::BETWEEN:
{
Item *fld;
if (!((Item_func_between*)cond)->negated &&
(fld= args[0]->real_item())->type() == Item::FIELD_ITEM &&
args[1]->eq(args[2], ((Item_field*)fld)->field->binary()))
{
if (add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1],
usable_tables) ||
add_eq_dep(te, fdeps, *and_level, cond_func, args[1], args[0],
usable_tables))
return TRUE;
}
break;
}
case Item_func::EQ_FUNC:
case Item_func::EQUAL_FUNC:
{
add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1],
usable_tables);
add_eq_dep(te, fdeps, *and_level, cond_func, args[1], args[0],
usable_tables);
break;
}
case Item_func::ISNULL_FUNC:
{
Item *tmp=new Item_null;
if (!tmp || add_eq_dep(te, fdeps, *and_level, cond_func, args[0], args[1],
usable_tables))
return TRUE;
break;
}
case Item_func::MULT_EQUAL_FUNC:
{
Item_equal *item_equal= (Item_equal *) cond;
Item *const_item= item_equal->get_const();
Item_equal_iterator it(*item_equal);
Item_field *item;
if (const_item)
{
/*
For each field field1 from item_equal consider the equality
field1=const_item as a condition allowing an index access of the table
with field1 by the keys value of field1.
*/
while ((item= it++))
{
if (add_eq_dep(te, fdeps, *and_level, cond_func, item, const_item,
usable_tables))
return TRUE;
}
}
else
{
/*
Consider all pairs of different fields included into item_equal.
For each of them (field1, field1) consider the equality
field1=field2 as a condition allowing an index access of the table
with field1 by the keys value of field2.
*/
Item_equal_iterator fi(*item_equal);
while ((item= fi++))
{
Field *field= item->field;
Item_field *item2;
while ((item2= it++))
{
if (!field->eq(item2->field))
{
if (add_eq_dep(te, fdeps, *and_level, cond_func, item, item2,
usable_tables))
return TRUE;
}
}
it.rewind();
}
}
break;
}
default:
break;
}
return FALSE;
}
/*
Perform an OR operation on two (adjacent) Equality_module arrays.
SYNOPSIS
merge_func_deps()
start Start of left OR-part
new_fields Start of right OR-part
end End of right OR-part
and_level AND-level.
DESCRIPTION
This function is invoked for two adjacent arrays of Equality_module elements:
$LEFT_PART $RIGHT_PART
+-----------------------+-----------------------+
start new_fields end
The goal is to produce an array which would correspnd to the combined
$LEFT_PART OR $RIGHT_PART
condition. This is achieved as follows: First, we apply distrubutive law:
(fdep_A_1 AND fdep_A_2 AND ...) OR (fdep_B_1 AND fdep_B_2 AND ...) =
= AND_ij (fdep_A_[i] OR fdep_B_[j])
Then we walk over the obtained "fdep_A_[i] OR fdep_B_[j]" pairs, and
- Discard those that that have left and right part referring to different
columns. We can't infer anything useful from "col1=expr1 OR col2=expr2".
- When left and right parts refer to the same column, we check if they are
essentially the same.
= If they are the same, we keep one copy
"t.col=expr OR t.col=expr" -> "t.col=expr
= if they are different , then we discard both
"t.col=expr1 OR t.col=expr2" -> (nothing useful)
(no per-table or for-index FUNC_DEPS exist yet at this phase).
See also merge_key_fields().
RETURN
End of the result array
*/
static
Equality_module *merge_func_deps(Equality_module *start, Equality_module *new_fields,
Equality_module *end, uint and_level)
{
if (start == new_fields)
return start; // Impossible or
if (new_fields == end)
return start; // No new fields, skip all
Equality_module *first_free=new_fields;
for (; new_fields != end ; new_fields++)
{
for (Equality_module *old=start ; old != first_free ; old++)
{
/*
TODO: does it make sense to attempt to merging multiple-equalities?
A: YES.
(a=b=c) OR (a=b=d) produce "a=b".
QQ:
What to use for merging? Trivial N*M algorithm or pre-sort and then
merge ordered sequences?
*/
if (old->field == new_fields->field)
{
if (!new_fields->expression->const_item())
{
/*
If the value matches, we can use the key reference.
If not, we keep it until we have examined all new values
*/
if (old->expression->eq(new_fields->expression, old->field->field->binary()))
{
old->level= and_level;
}
}
else if (old->expression->eq_by_collation(new_fields->expression,
old->field->field->binary(),
old->field->field->charset()))
{
old->level= and_level;
}
else
{
/* The expressions are different. */
if (old == --first_free) // If last item
break;
*old= *first_free; // Remove old value
old--; // Retry this value
}
}
}
}
/*
Ok, the results are within the [start, first_free) range, and the useful
elements have level==and_level. Now, lets remove all unusable elements:
*/
for (Equality_module *old=start ; old != first_free ;)
{
if (old->level != and_level)
{ // Not used in all levels
if (old == --first_free)
break;
*old= *first_free; // Remove old value
continue;
}
old++;
}
return first_free;
}
/*
Add an Equality_module element for a given predicate, if applicable
DESCRIPTION
This function is modeled after add_key_field().
*/
static
bool add_eq_dep(Table_elimination *te, Equality_module **eq_dep,
uint and_level, Item_func *cond, Item *left, Item *right,
table_map usable_tables)
{
if ((left->used_tables() & usable_tables) &&
!(right->used_tables() & RAND_TABLE_BIT) &&
left->real_item()->type() == Item::FIELD_ITEM)
{
Field *field= ((Item_field*)left->real_item())->field;
if (field->result_type() == STRING_RESULT)
{
if (right->result_type() != STRING_RESULT)
{
if (field->cmp_type() != right->result_type())
return FALSE;
}
else
{
/*
We can't use indexes if the effective collation
of the operation differ from the field collation.
*/
if (field->cmp_type() == STRING_RESULT &&
((Field_str*)field)->charset() != cond->compare_collation())
return FALSE;
}
}
(*eq_dep)->type= Module_dep::MODULE_EXPRESSION; //psergey-todo;
if (!((*eq_dep)->field= get_field_value(te, field)))
return TRUE;
(*eq_dep)->expression= right;
(*eq_dep)->level= and_level;
(*eq_dep)++;
}
return FALSE;
}
/*
Get a Table_value object for the given table, creating it if necessary.
*/
static Table_value *get_table_value(Table_elimination *te, TABLE *table)
{
Table_value *tbl_dep;
if (!(tbl_dep= new Table_value(table)))
return NULL;
Key_module **key_list= &(tbl_dep->keys);
/* Add dependencies for unique keys */
for (uint i=0; i < table->s->keys; i++)
{
KEY *key= table->key_info + i;
if ((key->flags & (HA_NOSAME | HA_END_SPACE_KEY)) == HA_NOSAME)
{
Key_module *key_dep= new Key_module(tbl_dep, i, key->key_parts);
*key_list= key_dep;
key_list= &(key_dep->next_table_key);
}
}
return te->table_deps[table->tablenr]= tbl_dep;
}
/*
Get a Field_value object for the given field, creating it if necessary
*/
static Field_value *get_field_value(Table_elimination *te, Field *field)
{
TABLE *table= field->table;
Table_value *tbl_dep;
/* First, get the table*/
if (!(tbl_dep= te->table_deps[table->tablenr]))
{
if (!(tbl_dep= get_table_value(te, table)))
return NULL;
}
/* Try finding the field in field list */
Field_value **pfield= &(tbl_dep->fields);
while (*pfield && (*pfield)->field->field_index < field->field_index)
{
pfield= &((*pfield)->next_table_field);
}
if (*pfield && (*pfield)->field->field_index == field->field_index)
return *pfield;
/* Create the field and insert it in the list */
Field_value *new_field= new Field_value(tbl_dep, field);
new_field->next_table_field= *pfield;
*pfield= new_field;
return new_field;
}
/*
Create an Outer_join_module object for the given outer join
DESCRIPTION
Outer_join_module objects for children (or further descendants) are always
created before the parents.
*/
static
Outer_join_module *get_outer_join_dep(Table_elimination *te,
// TABLE_LIST *outer_join,
table_map deps_map)
{
Outer_join_module *oj_dep;
if (!(oj_dep= new Outer_join_module(/*outer_join, */my_count_bits(deps_map))))
return NULL;
te->n_outer_joins++;
/*
Collect a bitmap fo tables that we depend on, and also set parent pointer
for descendant outer join elements.
*/
Table_map_iterator it(deps_map);
int idx;
while ((idx= it.next_bit()) != Table_map_iterator::BITMAP_END)
{
Table_value *table_dep;
if (!(table_dep= te->table_deps[idx]))
{
/*
We get here only when ON expression had no references to inner tables
and Table_map objects weren't created for them. This is a rare/
unimportant case so it's ok to do not too efficient searches.
*/
TABLE *table= NULL;
for (TABLE_LIST *tlist= te->join->select_lex->leaf_tables; tlist;
tlist=tlist->next_leaf)
{
if (tlist->table->tablenr == (uint)idx)
{
table=tlist->table;
break;
}
}
DBUG_ASSERT(table);
if (!(table_dep= get_table_value(te, table)))
return NULL;
}
/*
Walk from the table up to its embedding outer joins. The goal is to
find the least embedded outer join nest and set its parent pointer to
point to the newly created Outer_join_module.
to set the pointer of its near
*/
//if (!table_dep->outer_join_dep)
table_dep->outer_join_dep= oj_dep;
/*
else
{
Outer_join_module *oj= table_dep->outer_join_dep;
while (oj->parent)
oj= oj->parent;
if (oj != oj_dep)
oj->parent=oj_dep;
}
*/
}
return oj_dep;
}
/*
This is used to analyze expressions in "tbl.col=expr" dependencies so
that we can figure out which fields the expression depends on.
*/
class Field_dependency_setter : public Field_enumerator
{
public:
Field_dependency_setter(Table_elimination *te_arg): te(te_arg)
{}
void see_field(Field *field)
{
Table_value *tbl_dep;
if ((tbl_dep= te->table_deps[field->table->tablenr]))
{
for (Field_value *field_dep= tbl_dep->fields; field_dep;
field_dep= field_dep->next_table_field)
{
if (field->field_index == field_dep->field->field_index)
{
uint offs= field_dep->bitmap_offset + expr_offset;
if (!bitmap_is_set(&te->expr_deps, offs))
te->equality_deps[expr_offset].unknown_args++;
bitmap_set_bit(&te->expr_deps, offs);
return;
}
}
/*
We got here if didn't find this field. It's not a part of
a unique key, and/or there is no field=expr element for it.
Bump the dependency anyway, this will signal that this dependency
cannot be satisfied.
*/
te->equality_deps[expr_offset].unknown_args++;
}
}
Table_elimination *te;
/* Offset of the expression we're processing in the dependency bitmap */
uint expr_offset;
};
/*
Setup equality dependencies
SYNOPSIS
setup_equality_deps()
te Table elimination context
bound_deps_list OUT Start of linked list of elements that were found to
be bound (caller will use this to see if that
allows to declare further elements bound)
*/
static
bool setup_equality_deps(Table_elimination *te, Module_dep **bound_deps_list)
{
DBUG_ENTER("setup_equality_deps");
if (!te->n_equality_deps)
DBUG_RETURN(TRUE);
/*
Count Field_value objects and assign each of them a unique bitmap_offset.
*/
uint offset= 0;
for (Table_value **tbl_dep=te->table_deps;
tbl_dep < te->table_deps + MAX_TABLES;
tbl_dep++) // psergey-todo: TODO change to Table_map_iterator
{
if (*tbl_dep)
{
for (Field_value *field_dep= (*tbl_dep)->fields;
field_dep;
field_dep= field_dep->next_table_field)
{
field_dep->bitmap_offset= offset;
offset += te->n_equality_deps;
}
}
}
void *buf;
if (!(buf= current_thd->alloc(bitmap_buffer_size(offset))) ||
bitmap_init(&te->expr_deps, (my_bitmap_map*)buf, offset, FALSE))
{
DBUG_RETURN(TRUE);
}
bitmap_clear_all(&te->expr_deps);
/*
Analyze all "field=expr" dependencies, and have te->expr_deps encode
dependencies of expressions from fields.
Also collect a linked list of equalities that are bound.
*/
Module_dep *bound_dep= NULL;
Field_dependency_setter deps_setter(te);
for (Equality_module *eq_dep= te->equality_deps;
eq_dep < te->equality_deps + te->n_equality_deps;
eq_dep++)
{
deps_setter.expr_offset= eq_dep - te->equality_deps;
eq_dep->unknown_args= 0;
eq_dep->expression->walk(&Item::check_column_usage_processor, FALSE,
(uchar*)&deps_setter);
if (!eq_dep->unknown_args)
{
eq_dep->next= bound_dep;
bound_dep= eq_dep;
}
}
*bound_deps_list= bound_dep;
DBUG_EXECUTE("test", dbug_print_deps(te); );
DBUG_RETURN(FALSE);
}
/*
Perform table elimination
SYNOPSIS
eliminate_tables()
join Join to work on
const_tbl_count INOUT Number of constant tables (this includes
eliminated tables)
const_tables INOUT Bitmap of constant tables
DESCRIPTION
This function is the entry point for table elimination.
The idea behind table elimination is that if we have an outer join:
SELECT * FROM t1 LEFT JOIN
(t2 JOIN t3) ON t3.primary_key=t1.col AND
t4.primary_key=t2.col
such that
1. columns of the inner tables are not used anywhere ouside the outer
join (not in WHERE, not in GROUP/ORDER BY clause, not in select list
etc etc), and
2. inner side of the outer join is guaranteed to produce at most one
record combination for each record combination of outer tables.
then the inner side of the outer join can be removed from the query.
This is because it will always produce one matching record (either a
real match or a NULL-complemented record combination), and since there
are no references to columns of the inner tables anywhere, it doesn't
matter which record combination it was.
This function primary handles checking #1. It collects a bitmap of
tables that are not used in select list/GROUP BY/ORDER BY/HAVING/etc and
thus can possibly be eliminated.
SIDE EFFECTS
See the OVERVIEW section at the top of this file.
*/
static uint
eliminate_tables_for_list(Table_elimination *te,
List<TABLE_LIST> *join_list,
Item *on_expr,
table_map tables_in_list,
table_map tables_used_elsewhere);
void eliminate_tables(JOIN *join)
{
THD* thd= join->thd;
Item *item;
table_map used_tables;
DBUG_ENTER("eliminate_tables");
DBUG_ASSERT(join->eliminated_tables == 0);
/* If there are no outer joins, we have nothing to eliminate: */
if (!join->outer_join)
DBUG_VOID_RETURN;
/* Find the tables that are referred to from WHERE/HAVING */
used_tables= (join->conds? join->conds->used_tables() : 0) |
(join->having? join->having->used_tables() : 0);
/* Add tables referred to from the select list */
List_iterator<Item> it(join->fields_list);
while ((item= it++))
used_tables |= item->used_tables();
/* Add tables referred to from ORDER BY and GROUP BY lists */
ORDER *all_lists[]= { join->order, join->group_list};
for (int i=0; i < 2; i++)
{
for (ORDER *cur_list= all_lists[i]; cur_list; cur_list= cur_list->next)
used_tables |= (*(cur_list->item))->used_tables();
}
if (join->select_lex == &thd->lex->select_lex)
{
/* Multi-table UPDATE and DELETE: don't eliminate the tables we modify: */
used_tables |= thd->table_map_for_update;
/* Multi-table UPDATE: don't eliminate tables referred from SET statement */
if (thd->lex->sql_command == SQLCOM_UPDATE_MULTI)
{
List_iterator<Item> it2(thd->lex->value_list);
while ((item= it2++))
used_tables |= item->used_tables();
}
}
table_map all_tables= join->all_tables_map();
if (all_tables & ~used_tables)
{
/* There are some tables that we probably could eliminate. Try it. */
Table_elimination te(join);
uint m= max(thd->lex->current_select->max_equal_elems,1);
uint max_elems= ((thd->lex->current_select->cond_count+1)*2 +
thd->lex->current_select->between_count)*m + 1 + 10;
if (!(te.equality_deps= new Equality_module[max_elems]))
DBUG_VOID_RETURN;
//Equality_module *eq_deps_end= te.equality_deps;
//table_map eliminable_tables= 0;
/*
if (collect_funcdeps_for_join_list(&te, join->join_list,
FALSE,
used_tables,
&eliminable_tables,
&eq_deps))
DBUG_VOID_RETURN;
*/
eliminate_tables_for_list(&te, join->join_list,
NULL,
(table_map(1) << join->tables) - 1,
used_tables);
/*te.n_equality_deps= eq_deps_end - te.equality_deps;
Module_dep *bound_modules;
//Value_dep *bound_values;
if (setup_equality_deps(&te, &bound_modules))
DBUG_VOID_RETURN;
run_elimination_wave(&te, bound_modules);
*/
}
DBUG_VOID_RETURN;
}
////////////////////////////
/*
Perform table elimination in a given join list
SYNOPSIS
eliminate_tables_for_list()
join The join
leaves_arr OUT Store here an array of leaf (base) tables that
are descendants of the join_list, and increment
the pointer to point right above the array.
join_list Join list to work on
its_outer_join TRUE <=> join_list is an inner side of an outer
join
FALSE <=> otherwise (this is top-level join list)
tables_in_list Bitmap of tables embedded in the join_list.
tables_used_elsewhere Bitmap of tables that are referred to from
somewhere outside of the join list (e.g.
select list, HAVING, etc).
DESCRIPTION
Perform table elimination for a join list.
Try eliminating children nests first.
The "all tables in join nest can produce only one matching record
combination" property checking is modeled after constant table detection,
plus we reuse info attempts to eliminate child join nests.
RETURN
Number of children left after elimination. 0 means everything was
eliminated.
*/
bool try_eliminating(Table_elimination *te, table_map tables_in_list,
Item* on_expr)
{
uint and_level=0;
Equality_module* eq_dep= te->equality_deps;
build_eq_deps_for_cond(te, &eq_dep, &and_level, on_expr,
tables_in_list);
te->n_equality_deps= eq_dep - te->equality_deps;
Module_dep *bound_modules;
get_outer_join_dep(te, tables_in_list);
if (!setup_equality_deps(te, &bound_modules) &&
run_elimination_wave(te, bound_modules))
return TRUE; // eliminated
return FALSE;
}
static uint
eliminate_tables_for_list(Table_elimination *te, List<TABLE_LIST> *join_list,
Item *on_expr,
table_map tables_in_list,
table_map tables_used_elsewhere)
{
TABLE_LIST *tbl;
List_iterator<TABLE_LIST> it(*join_list);
table_map tables_used_on_left= 0;
bool not_eliminated= FALSE;
while ((tbl= it++))
{
if (tbl->on_expr)
{
table_map outside_used_tables= tables_used_elsewhere |
tables_used_on_left;
if (tbl->nested_join)
{
/* This is "... LEFT JOIN (join_nest) ON cond" */
if (eliminate_tables_for_list(te,
&tbl->nested_join->join_list,
tbl->on_expr,
tbl->nested_join->used_tables,
outside_used_tables))
{
mark_as_eliminated(te->join, tbl);
}
else
not_eliminated= TRUE;
}
else
{
/* This is "... LEFT JOIN tbl ON cond" */
if (!(tbl->table->map & outside_used_tables) &&
try_eliminating(te, tbl->table->map, tbl->on_expr))
{
mark_as_eliminated(te->join, tbl);
}
else
not_eliminated= TRUE;
}
tables_used_on_left |= tbl->on_expr->used_tables();
}
else
{
DBUG_ASSERT(!tbl->nested_join);
}
}
/* Try eliminating the nest we're called for */
if (!not_eliminated && on_expr && !(tables_in_list & tables_used_elsewhere))
return try_eliminating(te, tables_in_list & ~te->join->eliminated_tables,
on_expr);
return FALSE; /* not eliminated */
}
#if 0
/*
Build functional dependency graph for elements of given join list
SYNOPSIS
collect_funcdeps_for_join_list()
te Table elimination context.
join_list Join list to work on
build_eq_deps TRUE <=> build Equality_module elements for all
members of the join list, even if they cannot
be individually eliminated
tables_used_elsewhere Bitmap of tables that are referred to from
somewhere outside of this join list (e.g.
select list, HAVING, ON expressions of parent
joins, etc).
eliminable_tables INOUT Tables that can potentially be eliminated
(needed so we know for which tables to build
dependencies for)
eq_dep INOUT End of array of equality dependencies.
DESCRIPTION
.
*/
static bool
collect_funcdeps_for_join_list(Table_elimination *te,
List<TABLE_LIST> *join_list,
bool build_eq_deps,
table_map tables_used_elsewhere,
table_map *eliminable_tables,
Equality_module **eq_dep)
{
TABLE_LIST *tbl;
List_iterator<TABLE_LIST> it(*join_list);
table_map tables_used_on_left= 0;
while ((tbl= it++))
{
if (tbl->on_expr)
{
table_map outside_used_tables= tables_used_elsewhere |
tables_used_on_left;
bool eliminable;
table_map cur_map;
if (tbl->nested_join)
{
/* This is "... LEFT JOIN (join_nest) ON cond" */
cur_map= tbl->nested_join->used_tables;
eliminable= !(cur_map & outside_used_tables);
if (eliminable)
*eliminable_tables |= cur_map;
if (collect_funcdeps_for_join_list(te, &tbl->nested_join->join_list,
eliminable || build_eq_deps,
outside_used_tables,
eliminable_tables,
eq_dep))
return TRUE;
}
else
{
/* This is "... LEFT JOIN tbl ON cond" */
cur_map= tbl->table->map;
eliminable= !(tbl->table->map & outside_used_tables);
*eliminable_tables |= cur_map;
}
if (eliminable || build_eq_deps)
{
// build comp_cond from ON expression
uint and_level=0;
build_eq_deps_for_cond(te, eq_dep, &and_level, tbl->on_expr,
*eliminable_tables);
}
if (eliminable && !get_outer_join_dep(te, tbl, cur_map))
return TRUE;
tables_used_on_left |= tbl->on_expr->used_tables();
}
}
return FALSE;
}
#endif
////////////////////////////
static
void signal_from_field_to_exprs(Table_elimination* te, Field_value *field_dep,
Module_dep **bound_modules)
{
for (uint i=0; i < te->n_equality_deps; i++)
{
if (bitmap_is_set(&te->expr_deps, field_dep->bitmap_offset + i) &&
te->equality_deps[i].unknown_args &&
!--te->equality_deps[i].unknown_args)
{
/* Mark as bound and add to the list */
Equality_module* eq_dep= &te->equality_deps[i];
eq_dep->next= *bound_modules;
*bound_modules= eq_dep;
}
}
}
static
bool run_elimination_wave(Table_elimination *te, Module_dep *bound_modules)
{
Value_dep *bound_values= NULL;
/*
Run the wave.
All Func_dep-derived objects are divided into three classes:
- Those that have bound=FALSE
- Those that have bound=TRUE
- Those that have bound=TRUE and are in the list..
*/
while (bound_modules)
{
for (;bound_modules; bound_modules= bound_modules->next)
{
switch (bound_modules->type)
{
case Module_dep::MODULE_EXPRESSION:
{
/* It's a field=expr and we got to know the expr, so we know the field */
Equality_module *eq_dep= (Equality_module*)bound_modules;
if (!eq_dep->field->bound)
{
/* Mark as bound and add to the list */
eq_dep->field->bound= TRUE;
eq_dep->field->next= bound_values;
bound_values= eq_dep->field;
}
break;
}
case Module_dep::MODULE_UNIQUE_KEY:
{
/* Unique key is known means the table is known */
Table_value *table_dep=((Key_module*)bound_modules)->table;
if (!table_dep->bound)
{
/* Mark as bound and add to the list */
table_dep->bound= TRUE;
table_dep->next= bound_values;
bound_values= table_dep;
}
break;
}
case Module_dep::MODULE_OUTER_JOIN:
{
//Outer_join_module *outer_join_dep= (Outer_join_module*)bound_modules;
//mark_as_eliminated(te->join, outer_join_dep->table_list);
//if (!--te->n_outer_joins)
{
DBUG_PRINT("info", ("Table elimination eliminated everything"
" it theoretically could"));
return TRUE;
}
break;
}
case Module_dep::MODULE_MULTI_EQUALITY:
default:
DBUG_ASSERT(0);
}
}
for (;bound_values; bound_values=bound_values->next)
{
switch (bound_values->type)
{
case Value_dep::VALUE_FIELD:
{
/*
Field became known. Check out
- unique keys we belong to
- expressions that depend on us.
*/
Field_value *field_dep= (Field_value*)bound_values;
DBUG_PRINT("info", ("field %s.%s is now bound",
field_dep->field->table->alias,
field_dep->field->field_name));
for (Key_module *key_dep= field_dep->table->keys; key_dep;
key_dep= key_dep->next_table_key)
{
if (field_dep->field->part_of_key.is_set(key_dep->keyno) &&
key_dep->unknown_args && !--key_dep->unknown_args)
{
DBUG_PRINT("info", ("key %s.%s is now bound",
key_dep->table->table->alias,
key_dep->table->table->key_info[key_dep->keyno].name));
/* Mark as bound and add to the list */
key_dep->next= bound_modules;
bound_modules= key_dep;
}
}
signal_from_field_to_exprs(te, field_dep, &bound_modules);
break;
}
case Value_dep::VALUE_TABLE:
{
Table_value *table_dep=(Table_value*)bound_values;
DBUG_PRINT("info", ("table %s is now bound",
table_dep->table->alias));
/*
Table is known means
- all its fields are known
- one more element in outer join nest is known
*/
for (Field_value *field_dep= table_dep->fields; field_dep;
field_dep= field_dep->next_table_field)
{
if (!field_dep->bound)
{
/* Mark as bound and add to the list */
field_dep->bound= TRUE;
signal_from_field_to_exprs(te, field_dep, &bound_modules);
}
}
//for (
Outer_join_module *outer_join_dep= table_dep->outer_join_dep;
// outer_join_dep; outer_join_dep= outer_join_dep->parent)
{
if (outer_join_dep->unknown_args &&
!--outer_join_dep->unknown_args)
{
/* Mark as bound and add to the list */
outer_join_dep->next= bound_modules;
bound_modules= outer_join_dep;
}
}
break;
}
default:
DBUG_ASSERT(0);
}
}
}
return FALSE;
}
/*
Mark one table or the whole join nest as eliminated.
*/
static void mark_as_eliminated(JOIN *join, TABLE_LIST *tbl)
{
TABLE *table;
/*
NOTE: there are TABLE_LIST object that have
tbl->table!= NULL && tbl->nested_join!=NULL and
tbl->table == tbl->nested_join->join_list->element(..)->table
*/
if (tbl->nested_join)
{
TABLE_LIST *child;
List_iterator<TABLE_LIST> it(tbl->nested_join->join_list);
while ((child= it++))
mark_as_eliminated(join, child);
}
else if ((table= tbl->table))
{
JOIN_TAB *tab= tbl->table->reginfo.join_tab;
if (!(join->const_table_map & tab->table->map))
{
DBUG_PRINT("info", ("Eliminated table %s", table->alias));
tab->type= JT_CONST;
join->eliminated_tables |= table->map;
join->const_table_map|= table->map;
set_position(join, join->const_tables++, tab, (KEYUSE*)0);
}
}
if (tbl->on_expr)
tbl->on_expr->walk(&Item::mark_as_eliminated_processor, FALSE, NULL);
}
#ifndef DBUG_OFF
static
void dbug_print_deps(Table_elimination *te)
{
DBUG_ENTER("dbug_print_deps");
DBUG_LOCK_FILE;
fprintf(DBUG_FILE,"deps {\n");
/* Start with printing equalities */
for (Equality_module *eq_dep= te->equality_deps;
eq_dep != te->equality_deps + te->n_equality_deps; eq_dep++)
{
char buf[128];
String str(buf, sizeof(buf), &my_charset_bin);
str.length(0);
eq_dep->expression->print(&str, QT_ORDINARY);
fprintf(DBUG_FILE, " equality%d: %s -> %s.%s\n",
eq_dep - te->equality_deps,
str.c_ptr(),
eq_dep->field->table->table->alias,
eq_dep->field->field->field_name);
}
fprintf(DBUG_FILE,"\n");
/* Then tables and their fields */
for (uint i=0; i < MAX_TABLES; i++)
{
Table_value *table_dep;
if ((table_dep= te->table_deps[i]))
{
/* Print table */
fprintf(DBUG_FILE, " table %s\n", table_dep->table->alias);
/* Print fields */
for (Field_value *field_dep= table_dep->fields; field_dep;
field_dep= field_dep->next_table_field)
{
fprintf(DBUG_FILE, " field %s.%s ->", table_dep->table->alias,
field_dep->field->field_name);
uint ofs= field_dep->bitmap_offset;
for (uint bit= ofs; bit < ofs + te->n_equality_deps; bit++)
{
if (bitmap_is_set(&te->expr_deps, bit))
fprintf(DBUG_FILE, " equality%d ", bit - ofs);
}
fprintf(DBUG_FILE, "\n");
}
}
}
fprintf(DBUG_FILE,"\n}\n");
DBUG_UNLOCK_FILE;
DBUG_VOID_RETURN;
}
#endif
/**
@} (end of group Table_Elimination)
*/
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