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mariadb/sql/opt_sum.cc
stewart@willster.(none) 3ecc09e0e4 Merge willster.(none):/home/stewart/Documents/MySQL/4.1/main 
into  willster.(none):/home/stewart/Documents/MySQL/4.1/bug19914-mk2
2006-09-25 14:48:39 +10:00

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/* Copyright (C) 2000-2003 MySQL AB
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
/*
Optimising of MIN(), MAX() and COUNT(*) queries without 'group by' clause
by replacing the aggregate expression with a constant.
Given a table with a compound key on columns (a,b,c), the following
types of queries are optimised (assuming the table handler supports
the required methods)
SELECT COUNT(*) FROM t1[,t2,t3,...]
SELECT MIN(b) FROM t1 WHERE a=const
SELECT MAX(c) FROM t1 WHERE a=const AND b=const
SELECT MAX(b) FROM t1 WHERE a=const AND b<const
SELECT MIN(b) FROM t1 WHERE a=const AND b>const
SELECT MIN(b) FROM t1 WHERE a=const AND b BETWEEN const AND const
SELECT MAX(b) FROM t1 WHERE a=const AND b BETWEEN const AND const
Instead of '<' one can use '<=', '>', '>=' and '=' as well.
Instead of 'a=const' the condition 'a IS NULL' can be used.
If all selected fields are replaced then we will also remove all
involved tables and return the answer without any join. Thus, the
following query will be replaced with a row of two constants:
SELECT MAX(b), MIN(d) FROM t1,t2
WHERE a=const AND b<const AND d>const
(assuming a index for column d of table t2 is defined)
*/
#include "mysql_priv.h"
#include "sql_select.h"
static bool find_key_for_maxmin(bool max_fl, TABLE_REF *ref, Field* field,
COND *cond, uint *range_fl,
uint *key_prefix_length);
static int reckey_in_range(bool max_fl, TABLE_REF *ref, Field* field,
COND *cond, uint range_fl, uint prefix_len);
static int maxmin_in_range(bool max_fl, Field* field, COND *cond);
/*
Substitutes constants for some COUNT(), MIN() and MAX() functions.
SYNOPSIS
opt_sum_query()
tables Tables in query
all_fields All fields to be returned
conds WHERE clause
NOTE:
This function is only called for queries with sum functions and no
GROUP BY part.
RETURN VALUES
0 No errors
1 if all items were resolved
-1 on impossible conditions
OR an error number from my_base.h HA_ERR_... if a deadlock or a lock
wait timeout happens, for example
*/
int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
{
List_iterator_fast<Item> it(all_fields);
int const_result= 1;
bool recalc_const_item= 0;
longlong count= 1;
bool is_exact_count= TRUE;
table_map removed_tables= 0, outer_tables= 0, used_tables= 0;
table_map where_tables= 0;
Item *item;
int error;
if (conds)
where_tables= conds->used_tables();
/*
Analyze outer join dependencies, and, if possible, compute the number
of returned rows.
*/
for (TABLE_LIST *tl=tables; tl ; tl= tl->next)
{
/* Don't replace expression on a table that is part of an outer join */
if (tl->on_expr)
{
outer_tables|= tl->table->map;
/*
We can't optimise LEFT JOIN in cases where the WHERE condition
restricts the table that is used, like in:
SELECT MAX(t1.a) FROM t1 LEFT JOIN t2 join-condition
WHERE t2.field IS NULL;
*/
if (tl->table->map & where_tables)
return 0;
}
else
used_tables|= tl->table->map;
/*
If the storage manager of 'tl' gives exact row count, compute the total
number of rows. If there are no outer table dependencies, this count
may be used as the real count.
*/
if (tl->table->file->table_flags() & HA_NOT_EXACT_COUNT)
{
is_exact_count= FALSE;
count= 1; // ensure count != 0
}
else
{
error= tl->table->file->info(HA_STATUS_VARIABLE | HA_STATUS_NO_LOCK);
if(error)
{
tl->table->file->print_error(error, MYF(0));
return error;
}
count*= tl->table->file->records;
}
}
/*
Iterate through all items in the SELECT clause and replace
COUNT(), MIN() and MAX() with constants (if possible).
*/
while ((item= it++))
{
if (item->type() == Item::SUM_FUNC_ITEM)
{
Item_sum *item_sum= (((Item_sum*) item));
switch (item_sum->sum_func()) {
case Item_sum::COUNT_FUNC:
/*
If the expr in count(expr) can never be null we can change this
to the number of rows in the tables if this number is exact and
there are no outer joins.
*/
if (!conds && !((Item_sum_count*) item)->args[0]->maybe_null &&
!outer_tables && is_exact_count)
{
((Item_sum_count*) item)->make_const(count);
recalc_const_item= 1;
}
else
const_result= 0;
break;
case Item_sum::MIN_FUNC:
{
/*
If MIN(expr) is the first part of a key or if all previous
parts of the key is found in the COND, then we can use
indexes to find the key.
*/
Item *expr=item_sum->args[0];
if (expr->type() == Item::FIELD_ITEM)
{
byte key_buff[MAX_KEY_LENGTH];
TABLE_REF ref;
uint range_fl, prefix_len;
ref.key_buff= key_buff;
Item_field *item_field= ((Item_field*) expr);
TABLE *table= item_field->field->table;
/*
Look for a partial key that can be used for optimization.
If we succeed, ref.key_length will contain the length of
this key, while prefix_len will contain the length of
the beginning of this key without field used in MIN().
Type of range for the key part for this field will be
returned in range_fl.
*/
if (table->file->inited || (outer_tables & table->map) ||
!find_key_for_maxmin(0, &ref, item_field->field, conds,
&range_fl, &prefix_len))
{
const_result= 0;
break;
}
error= table->file->ha_index_init((uint) ref.key);
if (!ref.key_length)
error= table->file->index_first(table->record[0]);
else
error= table->file->index_read(table->record[0],key_buff,
ref.key_length,
range_fl & NEAR_MIN ?
HA_READ_AFTER_KEY :
HA_READ_KEY_OR_NEXT);
if (!error && reckey_in_range(0, &ref, item_field->field,
conds, range_fl, prefix_len))
error= HA_ERR_KEY_NOT_FOUND;
if (table->key_read)
{
table->key_read= 0;
table->file->extra(HA_EXTRA_NO_KEYREAD);
}
table->file->ha_index_end();
if (error)
{
if (error == HA_ERR_KEY_NOT_FOUND || error == HA_ERR_END_OF_FILE)
return -1; // No rows matching WHERE
/* HA_ERR_LOCK_DEADLOCK or some other error */
table->file->print_error(error, MYF(0));
return(error);
}
removed_tables|= table->map;
}
else if (!expr->const_item() || !is_exact_count)
{
/*
The optimization is not applicable in both cases:
(a) 'expr' is a non-constant expression. Then we can't
replace 'expr' by a constant.
(b) 'expr' is a costant. According to ANSI, MIN/MAX must return
NULL if the query does not return any rows. Thus, if we are not
able to determine if the query returns any rows, we can't apply
the optimization and replace MIN/MAX with a constant.
*/
const_result= 0;
break;
}
if (!count)
{
/* If count == 0, then we know that is_exact_count == TRUE. */
((Item_sum_min*) item_sum)->clear(); /* Set to NULL. */
}
else
((Item_sum_min*) item_sum)->reset(); /* Set to the constant value. */
((Item_sum_min*) item_sum)->make_const();
recalc_const_item= 1;
break;
}
case Item_sum::MAX_FUNC:
{
/*
If MAX(expr) is the first part of a key or if all previous
parts of the key is found in the COND, then we can use
indexes to find the key.
*/
Item *expr=item_sum->args[0];
if (expr->type() == Item::FIELD_ITEM)
{
byte key_buff[MAX_KEY_LENGTH];
TABLE_REF ref;
uint range_fl, prefix_len;
ref.key_buff= key_buff;
Item_field *item_field= ((Item_field*) expr);
TABLE *table= item_field->field->table;
/*
Look for a partial key that can be used for optimization.
If we succeed, ref.key_length will contain the length of
this key, while prefix_len will contain the length of
the beginning of this key without field used in MAX().
Type of range for the key part for this field will be
returned in range_fl.
*/
if (table->file->inited || (outer_tables & table->map) ||
!find_key_for_maxmin(1, &ref, item_field->field, conds,
&range_fl, &prefix_len))
{
const_result= 0;
break;
}
error= table->file->ha_index_init((uint) ref.key);
if (!ref.key_length)
error= table->file->index_last(table->record[0]);
else
error= table->file->index_read(table->record[0], key_buff,
ref.key_length,
range_fl & NEAR_MAX ?
HA_READ_BEFORE_KEY :
HA_READ_PREFIX_LAST_OR_PREV);
if (!error && reckey_in_range(1, &ref, item_field->field,
conds, range_fl, prefix_len))
error= HA_ERR_KEY_NOT_FOUND;
if (table->key_read)
{
table->key_read=0;
table->file->extra(HA_EXTRA_NO_KEYREAD);
}
table->file->ha_index_end();
if (error)
{
if (error == HA_ERR_KEY_NOT_FOUND || error == HA_ERR_END_OF_FILE)
return -1; // No rows matching WHERE
/* HA_ERR_LOCK_DEADLOCK or some other error */
table->file->print_error(error, MYF(0));
return(error);
}
removed_tables|= table->map;
}
else if (!expr->const_item() || !is_exact_count)
{
/*
The optimization is not applicable in both cases:
(a) 'expr' is a non-constant expression. Then we can't
replace 'expr' by a constant.
(b) 'expr' is a costant. According to ANSI, MIN/MAX must return
NULL if the query does not return any rows. Thus, if we are not
able to determine if the query returns any rows, we can't apply
the optimization and replace MIN/MAX with a constant.
*/
const_result= 0;
break;
}
if (!count)
{
/* If count != 1, then we know that is_exact_count == TRUE. */
((Item_sum_max*) item_sum)->clear(); /* Set to NULL. */
}
else
((Item_sum_max*) item_sum)->reset(); /* Set to the constant value. */
((Item_sum_max*) item_sum)->make_const();
recalc_const_item= 1;
break;
}
default:
const_result= 0;
break;
}
}
else if (const_result)
{
if (recalc_const_item)
item->update_used_tables();
if (!item->const_item())
const_result= 0;
}
}
/*
If we have a where clause, we can only ignore searching in the
tables if MIN/MAX optimisation replaced all used tables
We do not use replaced values in case of:
SELECT MIN(key) FROM table_1, empty_table
removed_tables is != 0 if we have used MIN() or MAX().
*/
if (removed_tables && used_tables != removed_tables)
const_result= 0; // We didn't remove all tables
return const_result;
}
/*
Test if the predicate compares a field with constants
SYNOPSIS
simple_pred()
func_item in: Predicate item
args out: Here we store the field followed by constants
inv_order out: Is set to 1 if the predicate is of the form 'const op field'
RETURN
0 func_item is a simple predicate: a field is compared with constants
1 Otherwise
*/
static bool simple_pred(Item_func *func_item, Item **args, bool *inv_order)
{
Item *item;
*inv_order= 0;
switch (func_item->argument_count()) {
case 1:
/* field IS NULL */
item= func_item->arguments()[0];
if (item->type() != Item::FIELD_ITEM)
return 0;
args[0]= item;
break;
case 2:
/* 'field op const' or 'const op field' */
item= func_item->arguments()[0];
if (item->type() == Item::FIELD_ITEM)
{
args[0]= item;
item= func_item->arguments()[1];
if (!item->const_item())
return 0;
args[1]= item;
}
else if (item->const_item())
{
args[1]= item;
item= func_item->arguments()[1];
if (item->type() != Item::FIELD_ITEM)
return 0;
args[0]= item;
*inv_order= 1;
}
else
return 0;
break;
case 3:
/* field BETWEEN const AND const */
item= func_item->arguments()[0];
if (item->type() == Item::FIELD_ITEM)
{
args[0]= item;
for (int i= 1 ; i <= 2; i++)
{
item= func_item->arguments()[i];
if (!item->const_item())
return 0;
args[i]= item;
}
}
else
return 0;
}
return 1;
}
/*
Check whether a condition matches a key to get {MAX|MIN}(field):
SYNOPSIS
matching_cond()
max_fl in: Set to 1 if we are optimising MAX()
ref in/out: Reference to the structure we store the key value
keyinfo in Reference to the key info
field_part in: Pointer to the key part for the field
cond in WHERE condition
key_part_used in/out: Map of matchings parts
range_fl in/out: Says whether including key will be used
prefix_len out: Length of common key part for the range
where MAX/MIN is searched for
DESCRIPTION
For the index specified by the keyinfo parameter, index that
contains field as its component (field_part), the function
checks whether the condition cond is a conjunction and all its
conjuncts referring to the columns of the same table as column
field are one of the following forms:
- f_i= const_i or const_i= f_i or f_i is null,
where f_i is part of the index
- field {<|<=|>=|>|=} const or const {<|<=|>=|>|=} field
- field between const1 and const2
RETURN
0 Index can't be used.
1 We can use index to get MIN/MAX value
*/
static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
KEY_PART_INFO *field_part, COND *cond,
key_part_map *key_part_used, uint *range_fl,
uint *prefix_len)
{
if (!cond)
return 1;
Field *field= field_part->field;
if (!(cond->used_tables() & field->table->map))
{
/* Condition doesn't restrict the used table */
return 1;
}
if (cond->type() == Item::COND_ITEM)
{
if (((Item_cond*) cond)->functype() == Item_func::COND_OR_FUNC)
return 0;
/* AND */
List_iterator_fast<Item> li(*((Item_cond*) cond)->argument_list());
Item *item;
while ((item= li++))
{
if (!matching_cond(max_fl, ref, keyinfo, field_part, item,
key_part_used, range_fl, prefix_len))
return 0;
}
return 1;
}
if (cond->type() != Item::FUNC_ITEM)
return 0; // Not operator, can't optimize
bool eq_type= 0; // =, <=> or IS NULL
bool noeq_type= 0; // < or >
bool less_fl= 0; // < or <=
bool is_null= 0;
bool between= 0;
switch (((Item_func*) cond)->functype()) {
case Item_func::ISNULL_FUNC:
is_null= 1; /* fall through */
case Item_func::EQ_FUNC:
case Item_func::EQUAL_FUNC:
eq_type= 1;
break;
case Item_func::LT_FUNC:
noeq_type= 1; /* fall through */
case Item_func::LE_FUNC:
less_fl= 1;
break;
case Item_func::GT_FUNC:
noeq_type= 1; /* fall through */
case Item_func::GE_FUNC:
break;
case Item_func::BETWEEN:
between= 1;
break;
default:
return 0; // Can't optimize function
}
Item *args[3];
bool inv;
/* Test if this is a comparison of a field and constant */
if (!simple_pred((Item_func*) cond, args, &inv))
return 0;
if (inv && !eq_type)
less_fl= 1-less_fl; // Convert '<' -> '>' (etc)
/* Check if field is part of the tested partial key */
byte *key_ptr= ref->key_buff;
KEY_PART_INFO *part;
for (part= keyinfo->key_part;
;
key_ptr+= part++->store_length)
{
if (part > field_part)
return 0; // Field is beyond the tested parts
if (part->field->eq(((Item_field*) args[0])->field))
break; // Found a part od the key for the field
}
bool is_field_part= part == field_part;
if (!(is_field_part || eq_type))
return 0;
key_part_map org_key_part_used= *key_part_used;
if (eq_type || between || max_fl == less_fl)
{
uint length= (key_ptr-ref->key_buff)+part->store_length;
if (ref->key_length < length)
/* Ultimately ref->key_length will contain the length of the search key */
ref->key_length= length;
if (!*prefix_len && part+1 == field_part)
*prefix_len= length;
if (is_field_part && eq_type)
*prefix_len= ref->key_length;
*key_part_used|= (key_part_map) 1 << (part - keyinfo->key_part);
}
if (org_key_part_used != *key_part_used ||
(is_field_part &&
(between || eq_type || max_fl == less_fl) && !cond->val_int()))
{
/*
It's the first predicate for this part or a predicate of the
following form that moves upper/lower bounds for max/min values:
- field BETWEEN const AND const
- field = const
- field {<|<=} const, when searching for MAX
- field {>|>=} const, when searching for MIN
*/
if (is_null)
{
part->field->set_null();
*key_ptr= (byte) 1;
}
else
{
store_val_in_field(part->field, args[between && max_fl ? 2 : 1],
CHECK_FIELD_IGNORE);
if (part->null_bit)
*key_ptr++= (byte) test(part->field->is_null());
part->field->get_key_image((char*) key_ptr, part->length,
part->field->charset(), Field::itRAW);
}
if (is_field_part)
{
if (between || eq_type)
*range_fl&= ~(NO_MAX_RANGE | NO_MIN_RANGE);
else
{
*range_fl&= ~(max_fl ? NO_MAX_RANGE : NO_MIN_RANGE);
if (noeq_type)
*range_fl|= (max_fl ? NEAR_MAX : NEAR_MIN);
else
*range_fl&= ~(max_fl ? NEAR_MAX : NEAR_MIN);
}
}
}
else if (eq_type)
{
if (!is_null && !cond->val_int() ||
is_null && !test(part->field->is_null()))
return 0; // Impossible test
}
else if (is_field_part)
*range_fl&= ~(max_fl ? NO_MIN_RANGE : NO_MAX_RANGE);
return 1;
}
/*
Check whether we can get value for {max|min}(field) by using a key.
SYNOPSIS
find_key_for_maxmin()
max_fl in: 0 for MIN(field) / 1 for MAX(field)
ref in/out Reference to the structure we store the key value
field in: Field used inside MIN() / MAX()
cond in: WHERE condition
range_fl out: Bit flags for how to search if key is ok
prefix_len out: Length of prefix for the search range
DESCRIPTION
If where condition is not a conjunction of 0 or more conjuct the
function returns false, otherwise it checks whether there is an
index including field as its k-th component/part such that:
1. for each previous component f_i there is one and only one conjunct
of the form: f_i= const_i or const_i= f_i or f_i is null
2. references to field occur only in conjucts of the form:
field {<|<=|>=|>|=} const or const {<|<=|>=|>|=} field or
field BETWEEN const1 AND const2
3. all references to the columns from the same table as column field
occur only in conjucts mentioned above.
4. each of k first components the index is not partial, i.e. is not
defined on a fixed length proper prefix of the field.
If such an index exists the function through the ref parameter
returns the key value to find max/min for the field using the index,
the length of first (k-1) components of the key and flags saying
how to apply the key for the search max/min value.
(if we have a condition field = const, prefix_len contains the length
of the whole search key)
NOTE
This function may set table->key_read to 1, which must be reset after
index is used! (This can only happen when function returns 1)
RETURN
0 Index can not be used to optimize MIN(field)/MAX(field)
1 Can use key to optimize MIN()/MAX()
In this case ref, range_fl and prefix_len are updated
*/
static bool find_key_for_maxmin(bool max_fl, TABLE_REF *ref,
Field* field, COND *cond,
uint *range_fl, uint *prefix_len)
{
if (!(field->flags & PART_KEY_FLAG))
return 0; // Not key field
TABLE *table= field->table;
uint idx= 0;
KEY *keyinfo,*keyinfo_end;
for (keyinfo= table->key_info, keyinfo_end= keyinfo+table->keys ;
keyinfo != keyinfo_end;
keyinfo++,idx++)
{
KEY_PART_INFO *part,*part_end;
key_part_map key_part_to_use= 0;
/*
Perform a check if index is not disabled by ALTER TABLE
or IGNORE INDEX.
*/
if (!table->keys_in_use_for_query.is_set(idx))
continue;
uint jdx= 0;
*prefix_len= 0;
for (part= keyinfo->key_part, part_end= part+keyinfo->key_parts ;
part != part_end ;
part++, jdx++, key_part_to_use= (key_part_to_use << 1) | 1)
{
if (!(table->file->index_flags(idx, jdx, 0) & HA_READ_ORDER))
return 0;
/* Check whether the index component is partial */
if (part->length < table->field[part->fieldnr-1]->pack_length())
break;
if (field->eq(part->field))
{
ref->key= idx;
ref->key_length= 0;
key_part_map key_part_used= 0;
*range_fl= NO_MIN_RANGE | NO_MAX_RANGE;
if (matching_cond(max_fl, ref, keyinfo, part, cond,
&key_part_used, range_fl, prefix_len) &&
!(key_part_to_use & ~key_part_used))
{
if (!max_fl && key_part_used == key_part_to_use && part->null_bit)
{
/*
SELECT MIN(key_part2) FROM t1 WHERE key_part1=const
If key_part2 may be NULL, then we want to find the first row
that is not null
*/
ref->key_buff[ref->key_length]= 1;
ref->key_length+= part->store_length;
*range_fl&= ~NO_MIN_RANGE;
*range_fl|= NEAR_MIN; // > NULL
}
/*
The following test is false when the key in the key tree is
converted (for example to upper case)
*/
if (field->part_of_key.is_set(idx))
{
table->key_read= 1;
table->file->extra(HA_EXTRA_KEYREAD);
}
return 1;
}
}
}
}
return 0;
}
/*
Check whether found key is in range specified by conditions
SYNOPSIS
reckey_in_range()
max_fl in: 0 for MIN(field) / 1 for MAX(field)
ref in: Reference to the key value and info
field in: Field used the MIN/MAX expression
cond in: WHERE condition
range_fl in: Says whether there is a condition to to be checked
prefix_len in: Length of the constant part of the key
RETURN
0 ok
1 WHERE was not true for the found row
*/
static int reckey_in_range(bool max_fl, TABLE_REF *ref, Field* field,
COND *cond, uint range_fl, uint prefix_len)
{
if (key_cmp_if_same(field->table, ref->key_buff, ref->key, prefix_len))
return 1;
if (!cond || (range_fl & (max_fl ? NO_MIN_RANGE : NO_MAX_RANGE)))
return 0;
return maxmin_in_range(max_fl, field, cond);
}
/*
Check whether {MAX|MIN}(field) is in range specified by conditions
SYNOPSIS
maxmin_in_range()
max_fl in: 0 for MIN(field) / 1 for MAX(field)
field in: Field used the MIN/MAX expression
cond in: WHERE condition
RETURN
0 ok
1 WHERE was not true for the found row
*/
static int maxmin_in_range(bool max_fl, Field* field, COND *cond)
{
/* If AND/OR condition */
if (cond->type() == Item::COND_ITEM)
{
List_iterator_fast<Item> li(*((Item_cond*) cond)->argument_list());
Item *item;
while ((item= li++))
{
if (maxmin_in_range(max_fl, field, item))
return 1;
}
return 0;
}
if (cond->used_tables() != field->table->map)
return 0;
bool less_fl= 0;
switch (((Item_func*) cond)->functype()) {
case Item_func::BETWEEN:
return cond->val_int() == 0; // Return 1 if WHERE is false
case Item_func::LT_FUNC:
case Item_func::LE_FUNC:
less_fl= 1;
case Item_func::GT_FUNC:
case Item_func::GE_FUNC:
{
Item *item= ((Item_func*) cond)->arguments()[1];
/* In case of 'const op item' we have to swap the operator */
if (!item->const_item())
less_fl= 1-less_fl;
/*
We only have to check the expression if we are using an expression like
SELECT MAX(b) FROM t1 WHERE a=const AND b>const
not for
SELECT MAX(b) FROM t1 WHERE a=const AND b<const
*/
if (max_fl != less_fl)
return cond->val_int() == 0; // Return 1 if WHERE is false
return 0;
}
case Item_func::EQ_FUNC:
case Item_func::EQUAL_FUNC:
break;
default: // Keep compiler happy
DBUG_ASSERT(1); // Impossible
break;
}
return 0;
}
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