Applies To
All Users
Summary
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If similar subqueries appear more than once in a sql, it is worth trying to rewrite the query using WITH subquery.
There are some things we should consider in order to get the benefit from it.
Solution
Subquery Factoring
Subquery Factoring or WITH clause enables you to assign a name to a subquery block that can then be referenced in multiple places in the query.
This subquery will be transformed as either a temporary table (materialization) or an inline view
Things to Consider when Deciding to Use Subquery Factoring
- Subquery Factoring can simplify the query statement making the format of the query clearer and more readable.
- As it eliminates the repeated scan of the same table, it may reduce physical I/O and logical reads.
This is more noticeable when retrieving a large amount of data. - It causes some recursive calls and "db block gets"(reads in current mode), and generates some redo data because it creates a temporary table during execution.
- It runs faster only if the total I/O is less than the plan that does not use materialization
Examples of Subquery Factoring
Example 1
SELECT ...
FROM A, B, C
WHERE ...
AND A.a in (select col from T1, T2, T3 where ...)
AND B.b in (select col from T1, T2, T3 where ...)
AND C.c in (select col from T1, T2, T3 where ...);
can be written as:
WITH S AS (select col from T1, T2, T3 where ...)
SELECT ...
FROM A, B, C
WHERE ...
AND A.a in (select col from S)
AND B.b in (select col from S)
AND C.c in (select col from S);
Example 2
SELECT ...
FROM (select ... from T1 ...) S1,
(select ... from T2 ...) S2,
(select ... from T3 ...) S3
WHERE ... ;
can be written as:
WITH S1 AS (select ... from T1 ...),
S2 AS (select ... from T2 ...),
S3 AS (select ... from T3 ...)
SELECT ...
FROM S1, S2, S3
WHERE ... ;
Influencing the Plan with Hints and Parameters
The decision whether to transfer the subquery into an inline view or a temporary table is done before the optimization and is determined by certain rules. However the decision can be influenced by including a hint in the WITH clause or by setting the parameter "_with_subquery".
Hints
-
/*+ MATERIALIZE */ : The subquery is materialized into a temporary table
WITH S AS (select /*+ MATERIALIZE */ col from T1, T2, T3 where ...)
SELECT ...
FROM A, B, C
WHERE ...
AND A.a in (select col from S)
AND B.b in (select col from S)
AND C.c in (select col from S); -
/*+ INLINE */ : The subquery is inlined and not materialized
WITH S AS (select /*+ INLINE */ col from T1, T2, T3 where ...)
SELECT ...
FROM A, B, C
WHERE ...
AND A.a in (select col from S)
AND B.b in (select col from S)
AND C.c in (select col from S);
Optimizer Parameter
The parameter "_WITH_SUBQUERY" can have the following values:
- optimizer : Let the optimizer choose (default)
- materialize: Always materialize the WITH subquery
- inline : Always inline.
The parameter can be set in pfile/spfile, at system level or at session level.
Demonstration
This test was done in SH demo schema.
Create 2 tables PRODUCTS2 and PRODUCTS3
connect sh/sh
create table products2 as select * from products;
create table products3 as select * from products;
Query 1 - Without Subquery Factoring
select P.prod_name, S.sale_count
from products P, (select prod_id, count(*) sale_count from sales group by prod_id) S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products2 P, (select prod_id, count(*) sale_count from sales group by prod_id) S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products3 P, (select prod_id, count(*) sale_count from sales group by prod_id) S
where P.prod_id = S.prod_id;
| | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop | |
|---|
| | 0 | SELECT STATEMENT | | 216 | 13536 | 862 (97)| 00:00:01 | | | |
| | 1 | UNION-ALL | | | | | | | | |
| |* 2 | HASH JOIN | | 72 | 4032 | 287 (90)| 00:00:01 | | | |
| | 3 | TABLE ACCESS FULL | PRODUCTS | 72 | 2160 | 2 (0)| 00:00:01 | | | |
| | 4 | VIEW | | 72 | 1872 | 285 (90)| 00:00:01 | | | |
| | 5 | HASH GROUP BY | | 72 | 288 | 285 (90)| 00:00:01 | | | |
| | 6 | PARTITION RANGE ALL | | 918K| 3589K| 29 (0)| 00:00:01 | 1 | 28 | |
| | 7 | BITMAP CONVERSION COUNT | | 918K| 3589K| 29 (0)| 00:00:01 | | | |
| | 8 | BITMAP INDEX FAST FULL SCAN| SALES_PROD_BIX | | | | | 1 | 28 | |
| |* 9 | HASH JOIN | | 72 | 4752 | 287 (90)| 00:00:01 | | | |
| | 10 | TABLE ACCESS FULL | PRODUCTS2 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 11 | VIEW | | 72 | 1872 | 285 (90)| 00:00:01 | | | |
| | 12 | HASH GROUP BY | | 72 | 288 | 285 (90)| 00:00:01 | | | |
| | 13 | PARTITION RANGE ALL | | 918K| 3589K| 29 (0)| 00:00:01 | 1 | 28 | |
| | 14 | BITMAP CONVERSION COUNT | | 918K| 3589K| 29 (0)| 00:00:01 | | | |
| | 15 | BITMAP INDEX FAST FULL SCAN| SALES_PROD_BIX | | | | | 1 | 28 | |
| |* 16 | HASH JOIN | | 72 | 4752 | 287 (90)| 00:00:01 | | | |
| | 17 | TABLE ACCESS FULL | PRODUCTS3 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 18 | VIEW | | 72 | 1872 | 285 (90)| 00:00:01 | | | |
| | 19 | HASH GROUP BY | | 72 | 288 | 285 (90)| 00:00:01 | | | |
| | 20 | PARTITION RANGE ALL | | 918K| 3589K| 29 (0)| 00:00:01 | 1 | 28 | |
| | 21 | BITMAP CONVERSION COUNT | | 918K| 3589K| 29 (0)| 00:00:01 | | | |
| | 22 | BITMAP INDEX FAST FULL SCAN| SALES_PROD_BIX | | | | | 1 | 28 | |
Predicate Information (identified by operation id):
2 - access("P"."PROD_ID"="S"."PROD_ID")
9 - access("P"."PROD_ID"="S"."PROD_ID")
16 - access("P"."PROD_ID"="S"."PROD_ID")
Statistics
0 recursive calls
0 db block gets
251 consistent gets
0 physical reads
0 redo size
9191 bytes sent via SQL*Net to client
573 bytes received via SQL*Net from client
16 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
216 rows processed
Query 1 - With Subquery Factoring
set autotrace on
with V_SALE
as (select prod_id, count(*) sale_count from sales group by prod_id)
select P.prod_name, S.sale_count
from products P, V_SALE S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products2 P, V_SALE S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products3 P, V_SALE S
where P.prod_id = S.prod_id;
| | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop | |
|---|
| | 0 | SELECT STATEMENT | | 216 | 13536 | 14 (72)| 00:00:01 | | | |
| | 1 | TEMP TABLE TRANSFORMATION | | | | | | | | |
| | 2 | LOAD AS SELECT | SYS_TEMP_0FD9D6608_1A3773F | | | | | | | |
| | 3 | HASH GROUP BY | | 72 | 288 | 285 (90)| 00:00:01 | | | |
| | 4 | PARTITION RANGE ALL | | 918K| 3589K| 29 (0)| 00:00:01 | 1 | 28 | |
| | 5 | BITMAP CONVERSION COUNT | | 918K| 3589K| 29 (0)| 00:00:01 | | | |
| | 6 | BITMAP INDEX FAST FULL SCAN| SALES_PROD_BIX | | | | | 1 | 28 | |
| | 7 | UNION-ALL | | | | | | | | |
| |* 8 | HASH JOIN | | 72 | 4032 | 5 (20)| 00:00:01 | | | |
| | 9 | TABLE ACCESS FULL | PRODUCTS | 72 | 2160 | 2 (0)| 00:00:01 | | | |
| | 10 | VIEW | | 72 | 1872 | 2 (0)| 00:00:01 | | | |
| | 11 | TABLE ACCESS FULL | SYS_TEMP_0FD9D6608_1A3773F | 72 | 288 | 2 (0)| 00:00:01 | | | |
| |* 12 | HASH JOIN | | 72 | 4752 | 5 (20)| 00:00:01 | | | |
| | 13 | TABLE ACCESS FULL | PRODUCTS2 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 14 | VIEW | | 72 | 1872 | 2 (0)| 00:00:01 | | | |
| | 15 | TABLE ACCESS FULL | SYS_TEMP_0FD9D6608_1A3773F | 72 | 288 | 2 (0)| 00:00:01 | | | |
| |* 16 | HASH JOIN | | 72 | 4752 | 5 (20)| 00:00:01 | | | |
| | 17 | TABLE ACCESS FULL | PRODUCTS3 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 18 | VIEW | | 72 | 1872 | 2 (0)| 00:00:01 | | | |
| | 19 | TABLE ACCESS FULL | SYS_TEMP_0FD9D6608_1A3773F | 72 | 288 | 2 (0)| 00:00:01 | | | |
Predicate Information (identified by operation id):
8 - access("P"."PROD_ID"="S"."PROD_ID")
12 - access("P"."PROD_ID"="S"."PROD_ID")
16 - access("P"."PROD_ID"="S"."PROD_ID")
Statistics
2 recursive calls
8 db block gets
114 consistent gets
1 physical reads
804 redo size
9191 bytes sent via SQL*Net to client
573 bytes received via SQL*Net from client
16 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
216 rows processed
Query 2 - Without Subquery Factoring
select P.prod_name, S.sale_count
from products P, (select prod_id, count(*) sale_count from sales where amount_sold <= 10 group by prod_id) S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products2 P, (select prod_id, count(*) sale_count from sales where amount_sold <= 10 group by prod_id) S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products3 P, (select prod_id, count(*) sale_count from sales where amount_sold <= 10 group by prod_id) S
where P.prod_id = S.prod_id;
| | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop | |
|---|
| | 0 | SELECT STATEMENT | | 216 | 13536 | 871 (78)| 00:00:01 | | | |
| | 1 | UNION-ALL | | | | | | | | |
| |* 2 | HASH JOIN | | 72 | 4032 | 290 (34)| 00:00:01 | | | |
| | 3 | TABLE ACCESS FULL | PRODUCTS | 72 | 2160 | 2 (0)| 00:00:01 | | | |
| | 4 | VIEW | | 72 | 1872 | 288 (34)| 00:00:01 | | | |
| | 5 | HASH GROUP BY | | 72 | 648 | 288 (34)| 00:00:01 | | | |
| | 6 | PARTITION RANGE ALL| | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
| |* 7 | TABLE ACCESS FULL | SALES | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
| |* 8 | HASH JOIN | | 72 | 4752 | 290 (34)| 00:00:01 | | | |
| | 9 | TABLE ACCESS FULL | PRODUCTS2 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 10 | VIEW | | 72 | 1872 | 288 (34)| 00:00:01 | | | |
| | 11 | HASH GROUP BY | | 72 | 648 | 288 (34)| 00:00:01 | | | |
| | 12 | PARTITION RANGE ALL| | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
| |* 13 | TABLE ACCESS FULL | SALES | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
| |* 14 | HASH JOIN | | 72 | 4752 | 290 (34)| 00:00:01 | | | |
| | 15 | TABLE ACCESS FULL | PRODUCTS3 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 16 | VIEW | | 72 | 1872 | 288 (34)| 00:00:01 | | | |
| | 17 | HASH GROUP BY | | 72 | 648 | 288 (34)| 00:00:01 | | | |
| | 18 | PARTITION RANGE ALL| | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
| |* 19 | TABLE ACCESS FULL | SALES | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
Predicate Information (identified by operation id):
2 - access("P"."PROD_ID"="S"."PROD_ID")
7 - filter("AMOUNT_SOLD"<=10)
8 - access("P"."PROD_ID"="S"."PROD_ID")
13 - filter("AMOUNT_SOLD"<=10)
14 - access("P"."PROD_ID"="S"."PROD_ID")
19 - filter("AMOUNT_SOLD"<=10)
Statistics
0 recursive calls
0 db block gets
4919 consistent gets
4857 physical reads
0 redo size
1675 bytes sent via SQL*Net to client
430 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
30 rows processed
Query 2 - With Subquery Factoring
with V_SALE
as (select prod_id, count(*) sale_count from sales where amount_sold <= 10 group by prod_id)
select P.prod_name, S.sale_count
from products P, V_SALE S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products2 P, V_SALE S
where P.prod_id = S.prod_id
union all
select P.prod_name, S.sale_count
from products3 P, V_SALE S
where P.prod_id = S.prod_id;
| | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop | |
|---|
| | 0 | SELECT STATEMENT | | 216 | 13536 | 14 (72)| 00:00:01 | | | |
| | 1 | TEMP TABLE TRANSFORMATION | | | | | | | | |
| | 2 | LOAD AS SELECT | SYS_TEMP_0FD9D660A_1A3773F | | | | | | | |
| | 3 | HASH GROUP BY | | 72 | 648 | 288 (34)| 00:00:01 | | | |
| | 4 | PARTITION RANGE ALL | | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
| |* 5 | TABLE ACCESS FULL | SALES | 2118 | 19062 | 287 (34)| 00:00:01 | 1 | 28 | |
| | 6 | UNION-ALL | | | | | | | | |
| |* 7 | HASH JOIN | | 72 | 4032 | 5 (20)| 00:00:01 | | | |
| | 8 | TABLE ACCESS FULL | PRODUCTS | 72 | 2160 | 2 (0)| 00:00:01 | | | |
| | 9 | VIEW | | 72 | 1872 | 2 (0)| 00:00:01 | | | |
| | 10 | TABLE ACCESS FULL | SYS_TEMP_0FD9D660A_1A3773F | 72 | 648 | 2 (0)| 00:00:01 | | | |
| |* 11 | HASH JOIN | | 72 | 4752 | 5 (20)| 00:00:01 | | | |
| | 12 | TABLE ACCESS FULL | PRODUCTS2 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 13 | VIEW | | 72 | 1872 | 2 (0)| 00:00:01 | | | |
| | 14 | TABLE ACCESS FULL | SYS_TEMP_0FD9D660A_1A3773F | 72 | 648 | 2 (0)| 00:00:01 | | | |
| |* 15 | HASH JOIN | | 72 | 4752 | 5 (20)| 00:00:01 | | | |
| | 16 | TABLE ACCESS FULL | PRODUCTS3 | 72 | 2880 | 2 (0)| 00:00:01 | | | |
| | 17 | VIEW | | 72 | 1872 | 2 (0)| 00:00:01 | | | |
| | 18 | TABLE ACCESS FULL | SYS_TEMP_0FD9D660A_1A3773F | 72 | 648 | 2 (0)| 00:00:01 | | | |
Predicate Information (identified by operation id):
5 - filter("AMOUNT_SOLD"<=10)
7 - access("P"."PROD_ID"="S"."PROD_ID")
11 - access("P"."PROD_ID"="S"."PROD_ID")
15 - access("P"."PROD_ID"="S"."PROD_ID")
Statistics
2 recursive calls
8 db block gets
1657 consistent gets
1620 physical reads
804 redo size
1675 bytes sent via SQL*Net to client
430 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
30 rows processed
Documentation
Oracle® Database SQL Language Reference
12c Release 1 (12.1)
subquery_factoring_clause
Restriction
Distributed transactions are not supported for temporary tables.
Query having WITH clause will be transformed as either a temporary table (materialization) or an inline view.
Note: The following fixes should be applied if query would be transformed as a temporary table (materialization):
<Document 9399589.8> "WITH" subqueries cannot be materialized inside a global transaction
9706532.8 "WITH" subqueries cannot be materialized inside a global transaction (or PLSQL RPC over DBlink)
如果使用变量，Oracle不知道with as 返回结果，可以使用
SELECT /*+ cardinality(mem,200000) */ 明确告知Oracle返回多少行，
