Performance differences between equal (=) and IN with one literal value

There are no big differences really, but if your column_value is indexed, IN operator may not read it as an index.

Encountered this problem once, so be careful.

There is no difference between those two statements, and the optimiser will transform the IN to the = when IN has just one element in it.

Though when you have a question like this, just run both statements, run their execution plan and see the differences. Here - you won't find any.

After a big search online, I found a document on SQL to support this (I assume it applies to all DBMS):

If there is only one value inside the parenthesis, this commend [sic] is equivalent to,

WHERE "column_name" = 'value1

Here is the execution plan of both queries in Oracle (most DBMS will process this the same):

EXPLAIN PLAN FOR
select * from dim_employees t
where t.identity_number = '123456789'

Plan hash value: 2312174735
-----------------------------------------------------
| Id  | Operation                   | Name          |
-----------------------------------------------------
|   0 | SELECT STATEMENT            |               |
|   1 |  TABLE ACCESS BY INDEX ROWID| DIM_EMPLOYEES |
|   2 |   INDEX UNIQUE SCAN         | SYS_C0029838  |
-----------------------------------------------------

And for IN() :

EXPLAIN PLAN FOR
select * from dim_employees t
where t.identity_number in('123456789');

Plan hash value: 2312174735
-----------------------------------------------------
| Id  | Operation                   | Name          |
-----------------------------------------------------
|   0 | SELECT STATEMENT            |               |
|   1 |  TABLE ACCESS BY INDEX ROWID| DIM_EMPLOYEES |
|   2 |   INDEX UNIQUE SCAN         | SYS_C0029838  |
-----------------------------------------------------

As you can see, both are identical. This is on an indexed column. Same goes for an unindexed column (just full table scan).

There is no difference when you are using it with a single value. If you will check the table scan, index scan, or index seek for the above two queries you will find that there is no difference between the two queries.

Is there any difference for same in Mysql and PostgresSQL?

No it would not have any difference on the two engines(Infact it would be same for most of the databases including SQL Server, Oracle etc). Both engines will convert IN to =