Question

Usually when I’m creating indexes on tables, I generally guess what the Fill Factor should be based on an educated guess of how the table will be used (many reads or many writes).

Is there a more scientific way to determine a more accurate Fill Factor value?

Answer 1

You could try running a big list of realistic operations and looking at IO queues for the different actions.

There are a lot of variables that govern it, such as the size of each row and the number of writes vs reads.

Basically: high fill factor = quicker read, low = quicker write.

However it’s not quite that simple, as almost all writes will be to a subset of rows that need to be looked up first.

For instance: set a fill factor to 10% and each single-row update will take 10 times as long to find the row it’s changing, even though a page split would then be very unlikely.

Generally you see fill factors 70% (very high write) to 95% (very high read).

It’s a bit of an art form.

I find that a good way of thinking of fill factors is as pages in an address book – the more tightly you pack the addresses the harder it is to change them, but the slimmer the book. I think I explained it better on my blog.