CTE vs subquery, which one is efficient? - sql

Question

SELECT col, (SELECT COUNT(*) FROM table) as total_count FROM table
This query executes subquery for every row, right?
Now if we have
;WITH CTE(total_count) AS (
SELECT COUNT(*) FROM table
)
SELECT col, (SELECT total_count FROM CTE) FROM table;
Will be second method more efficient? Will CTE execute COUNT(*) only once and then SELECT uses it as prepared value? or in second case also executed COUNT(*) for each row?

Answer 1

For Oracle the surest way is to observe the behaviour of the statements with extended statistics.
Do do so first increase the statistics level to ALL
alter session set statistics_level=all;
Then run both statements (fetching all rows) and find the SQL_ID of those statements
Finally display the statistics using following statement (passing the proper SQL_ID):
select * from table(dbms_xplan.display_cursor('your SQL_ID here',null,'ALLSTATS LAST'));
This gives for my test table
SQL_ID 5n0sdcu8347j9, child number 0
-------------------------------------
SELECT col, (SELECT COUNT(*) FROM t1) as total_count FROM t1
Plan hash value: 1306093980
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1000 |00:00:00.01 | 351 |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 338 |
| 2 | TABLE ACCESS FULL| T1 | 1 | 1061 | 1000 |00:00:00.01 | 338 |
| 3 | TABLE ACCESS FULL | T1 | 1 | 1061 | 1000 |00:00:00.01 | 351 |
-------------------------------------------------------------------------------------
and
SQL_ID fs0h660f08bj6, child number 0
-------------------------------------
WITH CTE(total_count) AS ( SELECT COUNT(*) FROM t1 ) SELECT col,
(SELECT total_count FROM CTE) FROM t1
Plan hash value: 1223456497
--------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1000 |00:00:00.01 | 351 |
| 1 | VIEW | | 1 | 1 | 1 |00:00:00.01 | 338 |
| 2 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 338 |
| 3 | TABLE ACCESS FULL| T1 | 1 | 1061 | 1000 |00:00:00.01 | 338 |
| 4 | TABLE ACCESS FULL | T1 | 1 | 1061 | 1000 |00:00:00.01 | 351 |
--------------------------------------------------------------------------------------
So the plans are slightly different, but in both cases the FULL TABLE SCAN is started only once (column Starts = 1). Which gives no real difference.
For purpose of camparison I run also a correlated subquery, which gives a complete different picture with high number of Starts (of FTS)
SQL_ID cbvwd6pm6699m, child number 0
-------------------------------------
SELECT col, (SELECT COUNT(*) FROM t1 where col = a.col) as total_count
FROM t1 a
Plan hash value: 1306093980
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1000 |00:00:00.01 | 351 |
| 1 | SORT AGGREGATE | | 1000 | 1 | 1000 |00:00:00.31 | 338K|
|* 2 | TABLE ACCESS FULL| T1 | 1000 | 11 | 1000 |00:00:00.31 | 338K|
| 3 | TABLE ACCESS FULL | T1 | 1 | 1061 | 1000 |00:00:00.01 | 351 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("COL"=:B1)

Answer 2

I believe that the query optimizer in both Oracle and SQL Server will recognize that the count query is not correlated, compute it once, and then use the cached result throughout the execution of the outer query.
Also, the CTE won't change anything as far as I know, since at execution time the code inside it will basically just be inlined into the actual outer query.
Here is a reference for Oracle which mentions that a non correlated subquery will be executed once and cached, except in cases where the outer query only has a few rows. In that case, it might not be cached because there isn't much of a penalty in executing the count subquery multiple times.