Question

I’ve be told and read it everywhere (but no one dared to explain why) that when composing an index on multiple columns I should put the most selective column first, for performance reasons.
Why is that?
Is it a myth?

Answer 1

I should put the most selective column first

According to Tom, column selectivity has no performance impact for queries that use all the columns in the index (it does affect Oracle’s ability to compress the index).

it is not the first thing, it is not the most important thing. sure, it is something to consider but it is relatively far down there in the grand scheme of things.

In certain strange, very peculiar and abnormal cases (like the above with really utterly skewed data), the selectivity could easily matter HOWEVER, they are

a) pretty rare
b) truly dependent on the values used at runtime, as all skewed queries are

so in general, look at the questions you have, try to minimize the indexes you need based on that.

The number of distinct values in a column in a concatenated index is not relevant when considering
the position in the index.

However, these considerations should come second when deciding on index column order. More importantly is to ensure that the index can be useful to many queries, so the column order has to reflect the use of those columns (or the lack thereof) in the where clauses of your queries (for the reason illustrated by AndreKR).

HOW YOU USE the index — that is what is relevant when deciding.

All other things being equal, I would still put the most selective column first. It just feels right…

Update: Another quote from Tom (thanks to milan for finding it).

In Oracle 5 (yes, version 5!), there was an argument for placing the most selective columns first
in an index.

Since then, it is not true that putting the most discriminating entries first in the index
will make the index smaller or more efficient. It seems like it will, but it will not.

With index
key compression, there is a compelling argument to go the other way since it can make the index
smaller. However, it should be driven by how you use the index, as previously stated.