Question

I was asked in an interview to enlighten the ways one can use to optimize the query Select * from TableA if it is taking a lot of time to execute. (TableA) can be any table with large amount of data. The interviewer didn’t leave me any option like to select few columns or to use “WHERE” clause rather he wanted me to give solution for the subject query.

Answer 1

It’s really hard to know what the interviewer was looking for.

They might be relatively inexperienced and expected answers like:

• “list all the columns instead of * since that’s way faster!”; or,
• “add an ORDER BY because that will always speed it up!”

The kinds of things an experienced person might be looking for are:

• inspect the query plan, are there computed columns or other similar things taking additional resources?
• revisit the requirements – do the users really need the whole table in arbitrary order?
• is there a clustered index on the table; if not, is the heap full of forwarding pointers?
• is there excessive fragmentation on the underlying table (and/or the index being used to satisfy the query)?
• is the query being blocked?
• what is the query waiting on?
• is the query waiting on an external resource (e.g. crappy I/O subsystem, a memory grant, a tempdb autogrow)?
• is the query parallel and suffering packet waits because the stats are out of date?

There are a lot of underlying things that may be making that query slow or that may make that query a bad choice.