How does SQL engines differ when we use equal sign and IN operator have same value? Does execution time changes?
1st one using equality check operator
WHERE column_value = 'All'
2nd one using IN operator and single value
WHERE column_value IN ('All')
Does SQL engine changes IN to = if only one value is there?
Is there any difference for same in MySQL and PostgreSQL?
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).