why useless statements in where clause slow down the sql query speed?

Question

I have two queries below:

SELECT cl.`cl_boolean`, l.`l_name`
FROM `card_legality` cl
LEFT JOIN `legality` l ON l.`legality_id` = cl.`legality_id`
WHERE cl.`card_id` = 23155


SELECT cl.`cl_boolean`, l.`l_name`
FROM `card_legality` cl
LEFT JOIN `legality` l ON l.`legality_id` = cl.`legality_id`
WHERE cl.`card_id` = 23155 or 1 = 2

(this is not the true case, just show the problem)

I want to know why the second one is so slow(almost 100 times slower in true case).

Okay, below is the query(oracle) in my case:

select *
from LA_TESTCASE this_ 
left outer join LA_RULE   rule1_    on this_.ROOTCAUSE_RULE_ID = rule1_.ID 
left outer join LA_TEST   test2_    on this_.TEST_ID = test2_.ID 
left outer join LA_SUITE  suite3_   on test2_.SUITE_ID = suite3_.ID 
left outer join LA_RUN    run4_     on suite3_.RUN_ID = run4_.ID
where (run4_.NAME = 'RRP_XO-245'/* or 1 = 2*/)
order by this_.ID desc;

It's almost the same as the sample case.

Answer 1

"It's almost the same as the sample case.". actually it's nothing like it.

in the first case, you were filtering on the left table of a left join. in the second, you are filtering on the right (outer joined table) of the query.

the presence of OR 1=2 in that case will most likely cause a full table scan to resolve it (again, run the explain plans to see this).

but your query makes no sense in that you are outer joining RUN4_ but then filtering on it in the WHERE clause (not the join itself).

run4_.NAME = 'RRP_XO-245'

you should tidy up the logic of the query.

Answer 2

In this case the cause is OR .

In the first query, the engine may use an index on card_id. It uses probably a hash join to join the two tables.

In the second the existence of OR cause the possibility of existence more rows that may not have card_id=23155. So, the index is useless: it should scan the entire table.

Also, in general or conditions are harder to put in hash joins, so it may be forced to do a NESTED LOOOPS JOIN.