Mysql optimize subquery in where clause - mysql

Question

If I execute a query e.g
SELECT *
FROM `table1`
WHERE 1 OR EXISTS (
SELECT show
FROM `table2`
WHERE id IN(1,2,3)
)
I would expect the optimiser to realise that the where clause allways resolves to true and that it is therefore unnecessary to run the subquery. As running the query with EXPLAIN shows this is not the case and the subquery is executed anyway.
This is a narrowed down example of a more complex problem where I tryed to execute different subqueries bases on a column value of an outer query like:
SELECT value FROM table t
LEFT JOIN...
WHERE
(SELECT
IF(t.value = 1,
(SELECT ...),
(SELECT ...)
)
)
The intension was that only one of the inners subqueries in the where condition is executed, but the same happens here, both get executed but only the value of one is used. So the result is correct but useless queries are run. I have tryed with CASE WHEN as well same problem. Not sure if it is because I using MariaDB or something I'm missing here.

Answer 1

Sorry, i hope that i understand your question correctly. In the first query with WHERE 1 MySQL NEVER executes the EXISTS subquery. EPLAIN only show you whats possible to do. You can see it simple with SHOW status LIKE 'Handler_read_first'; I have 2 tables. if i test with WHERE 1 its only increment by 1 (1 Table read) and without it increments by 2
sample
the tables
MariaDB [test]> select * from index_usage;
+----+------+------+------+-------+
| id | a | b | c | dummy |
+----+------+------+------+-------+
| 11 | 1 | 1 | 1 | a |
+----+------+------+------+-------+
1 row in set (0.00 sec)
MariaDB [test]> select * from index_usage_copy;
+----+------+------+------+-------+
| id | a | b | c | dummy |
+----+------+------+------+-------+
| 99 | 1 | 1 | 1 | a |
+----+------+------+------+-------+
1 row in set (0.00 sec)
the counter
MariaDB [test]> SHOW status LIKE 'Handler_read_first';
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Handler_read_first | 59 |
+--------------------+-------+
1 row in set (0.00 sec)
first query (without WHERE 1) increment by 2
MariaDB [test]> SELECT * from index_usage WHERE EXISTS ( SELECT 1 FROM index_usage_copy where id in (1,2,99)); SHOW status LIKE 'Handler_read_first';
+----+------+------+------+-------+
| id | a | b | c | dummy |
+----+------+------+------+-------+
| 11 | 1 | 1 | 1 | a |
+----+------+------+------+-------+
1 row in set (0.00 sec)
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Handler_read_first | 61 |
+--------------------+-------+
1 row in set (0.00 sec)
MariaDB [test]> SELECT * from index_usage WHERE EXISTS ( SELECT 1 FROM index_usage_copy where id in (1,2,99)); SHOW status LIKE 'Handler_read_first';
+----+------+------+------+-------+
| id | a | b | c | dummy |
+----+------+------+------+-------+
| 11 | 1 | 1 | 1 | a |
+----+------+------+------+-------+
1 row in set (0.00 sec)
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Handler_read_first | 63 |
+--------------------+-------+
1 row in set (0.00 sec)
MariaDB [test]> SELECT * from index_usage WHERE EXISTS ( SELECT 1 FROM index_usage_copy where id in (1,2,99)); SHOW status LIKE 'Handler_read_first';
+----+------+------+------+-------+
| id | a | b | c | dummy |
+----+------+------+------+-------+
| 11 | 1 | 1 | 1 | a |
+----+------+------+------+-------+
1 row in set (0.00 sec)
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Handler_read_first | 65 |
+--------------------+-------+
1 row in set (0.00 sec)
now with WHERE 1 - increments only by one
MariaDB [test]> SELECT * from index_usage WHERE 1 OR EXISTS ( SELECT 1 FROM index_usage_copy where id in (1,2,99)); SHOW status LIKE 'Handler_read_first';
+----+------+------+------+-------+
| id | a | b | c | dummy |
+----+------+------+------+-------+
| 11 | 1 | 1 | 1 | a |
+----+------+------+------+-------+
1 row in set (0.00 sec)
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Handler_read_first | 66 |
+--------------------+-------+
1 row in set (0.00 sec)
MariaDB [test]> SELECT * from index_usage WHERE 1 OR EXISTS ( SELECT 1 FROM index_usage_copy where id in (1,2,99)); SHOW status LIKE 'Handler_read_first';
+----+------+------+------+-------+
| id | a | b | c | dummy |
+----+------+------+------+-------+
| 11 | 1 | 1 | 1 | a |
+----+------+------+------+-------+
1 row in set (0.00 sec)
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Handler_read_first | 67 |
+--------------------+-------+
1 row in set (0.00 sec)
MariaDB [test]>

Answer 2

Also, EXPLAIN FORMAT=JSON says "optimized_away_subqueries" for 5.6 and 5.7. 8.0 and MariaDB 10.2 and 10.3 do not say that but seem to get rid of the subquery anyway.