This query is executed many times as part of an application, but runs slow:

```
SELECT [c2], [c1], [c5], [c4], [c3], [c6]
  from [MyBigTable]
 where [c7] = 1
   and [c3] in ('a','b','c','d','e')
   and [c4] is not NULL

```

It takes routinely more than 3 seconds, which is already bad, but sometimes the IN has 250 arguments, and the query takes more than one minute.
This is the estimated execution plan:

<https://www.brentozar.com/pastetheplan/?id=rJ-b2XalH>

Table has 11 millions rows.

* In about 11% of rows [c3] and [c4] are both not null.
* In about 88% of rows [c3] is null and [c4] is not null.
* In only 0.4% of rows [c3] is not null and [c4] is null.
* Both [c3] and [c4] can never be null at the same time.
* [c7] is 1 in 97% of cases

There is a nonclustered index on [c3], [c4], [c7], [c8] (in order) including [c2], [c5], [c6]

Every time the query has to be run, the PHP code prepares a SQL statement, executes once and discard. I am discussing with the coders whether this can be improved, but I don't hold my breath.

What can I do to improve the performances of the query?

Could a conditional index, "WHERE [c7]=1 AND [c4] is not NULL" seriously improve the situation?