MySQL - SELECT WHERE field IN (subquery) - Extreme

Question

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?

Answer 1

This is similar to my case, where I have a table named tabel_buku_besar. What I need are

1. Looking for record that have account_code='101.100' in tabel_buku_besar which have companyarea='20000' and also have IDR as currency

2. I need to get all record from tabel_buku_besar which have account_code same as step 1 but have transaction_number in step 1 result

while using select ... from...where....transaction_number in (select transaction_number from ....), my query running extremely slow and sometimes causing request time out or make my application not responding...

I try this combination and the result...not bad...

`select DATE_FORMAT(L.TANGGAL_INPUT,'%d-%m-%y') AS TANGGAL,
      L.TRANSACTION_NUMBER AS VOUCHER,
      L.ACCOUNT_CODE,
      C.DESCRIPTION,
      L.DEBET,
      L.KREDIT 
 from (select * from tabel_buku_besar A
                where A.COMPANYAREA='$COMPANYAREA'
                      AND A.CURRENCY='$Currency'
                      AND A.ACCOUNT_CODE!='$ACCOUNT'
                      AND (A.TANGGAL_INPUT BETWEEN STR_TO_DATE('$StartDate','%d/%m/%Y') AND STR_TO_DATE('$EndDate','%d/%m/%Y'))) L 
INNER JOIN (select * from tabel_buku_besar A
                     where A.COMPANYAREA='$COMPANYAREA'
                           AND A.CURRENCY='$Currency'
                           AND A.ACCOUNT_CODE='$ACCOUNT'
                           AND (A.TANGGAL_INPUT BETWEEN STR_TO_DATE('$StartDate','%d/%m/%Y') AND STR_TO_DATE('$EndDate','%d/%m/%Y'))) R ON R.TRANSACTION_NUMBER=L.TRANSACTION_NUMBER AND R.COMPANYAREA=L.COMPANYAREA 
LEFT OUTER JOIN master_account C ON C.ACCOUNT_CODE=L.ACCOUNT_CODE AND C.COMPANYAREA=L.COMPANYAREA 
ORDER BY L.TANGGAL_INPUT,L.TRANSACTION_NUMBER`

Answer 2

I have reformatted your slow sql query with www.prettysql.net

SELECT *
FROM some_table
WHERE
 relevant_field in
 (
  SELECT relevant_field
  FROM some_table
  GROUP BY relevant_field
  HAVING COUNT ( * ) > 1
 );

When using a table in both the query and the subquery, you should always alias both, like this:

SELECT *
FROM some_table as t1
WHERE
 t1.relevant_field in
 (
  SELECT t2.relevant_field
  FROM some_table as t2
  GROUP BY t2.relevant_field
  HAVING COUNT ( t2.relevant_field ) > 1
 );

Does that help?

Answer 3

Rewrite the query into this

SELECT st1.*, st2.relevant_field FROM sometable st1
INNER JOIN sometable st2 ON (st1.relevant_field = st2.relevant_field)
GROUP BY st1.id  /* list a unique sometable field here*/
HAVING COUNT(*) > 1

I think st2.relevant_field must be in the select, because otherwise the having clause will give an error, but I'm not 100% sure

Never use IN with a subquery; this is notoriously slow.
Only ever use IN with a fixed list of values.

More tips

1. If you want to make queries faster, don't do a SELECT * only select the fields that you really need.
2. Make sure you have an index on relevant_field to speed up the equi-join.
3. Make sure to group by on the primary key.
4. If you are on InnoDB and you only select indexed fields (and things are not too complex) than MySQL will resolve your query using only the indexes, speeding things way up.

General solution for 90% of your IN (select queries

Use this code

SELECT * FROM sometable a WHERE EXISTS (
  SELECT 1 FROM sometable b
  WHERE a.relevant_field = b.relevant_field
  GROUP BY b.relevant_field
  HAVING count(*) > 1) 

Answer 4

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
)