I've got a couple of duplicates in a database that I want to inspect, so what I did to see which are duplicates, I did this:
SELECT relevant_field
FROM some_table
GROUP BY relevant_field
HAVING COUNT(*) > 1
This way, I will get all rows with relevant_field occuring more than once. This query takes milliseconds to execute.
Now, I wanted to inspect each of the duplicates, so I thought I could SELECT each row in some_table with a relevant_field in the above query, so I did like this:
SELECT *
FROM some_table
WHERE relevant_field IN
(
SELECT relevant_field
FROM some_table
GROUP BY relevant_field
HAVING COUNT(*) > 1
)
This turns out to be extreeeemely slow for some reason (it takes minutes). What exactly is going on here to make it that slow? relevant_field is indexed.
Eventually I tried creating a view "temp_view" from the first query (SELECT relevant_field FROM some_table GROUP BY relevant_field HAVING COUNT(*) > 1)
, and then making my second query like this instead:
SELECT *
FROM some_table
WHERE relevant_field IN
(
SELECT relevant_field
FROM temp_view
)
And that works just fine. MySQL does this in some milliseconds.
Any SQL experts here who can explain what's going on?
The subquery is being run for each row because it is a correlated query. One can make a correlated query into a non-correlated query by selecting everything from the subquery, like so:
SELECT * FROM
(
SELECT relevant_field
FROM some_table
GROUP BY relevant_field
HAVING COUNT(*) > 1
) AS subquery
The final query would look like this:
SELECT *
FROM some_table
WHERE relevant_field IN
(
SELECT * FROM
(
SELECT relevant_field
FROM some_table
GROUP BY relevant_field
HAVING COUNT(*) > 1
) AS subquery
)