Home/ Questions/Q 5952651
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-22T17:42:41+00:00

Should I include col3 & col4 in my index on MyTable if this is

  • 0

Should I include col3 & col4 in my index on MyTable if this is the only query I intend to run on my database? 

Select MyTable.col3, MyTable.col4
From MyTable 
Inner Join MyOtherTable
On MyTable.col1 = MyOtherTable.col1
And MyTable.col2 = MyOtherTable.col2;

The tables I’m using have about half a million rows in them. For the purposes of my question, col1 & col2 are a unique set found in both tables.

Here’s the example table definition if you really need to know:

CREATE TABLE MyTable 
(col1 varchar(10), col2 varchar(10), col3 varchar(10), col4 varchar(10));

CREATE TABLE MyOtherTable 
(col1 varchar(10), col2 varchar(10));

So, should it be this?

   CREATE MyIdx ON MyTable (col1,col2);

Or this?

   CREATE MyIdx ON MyTable (col1,col2,col3,col4);
Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    adding columns col3 and col4 will not help because you’re just pulling those values after finding them using columns col1 and col2. The speed would normally come from making sure columns col1 and col2 are indexed.

    You should actually split those indexes since you’re not using them together:

    CREATE MyIdx ON MyTable (col1);
    CREATE MyIdx ON MyTable (col2);

    I don’t think a combined index will help you in this case.

    CORRECTION: I think I’ve misspoken, since you intend to use only that query on the two tables and never have the individual columns joined in isolation. In your case it appears you could get some speed up by putting them together. It would be interesting to benchmark this to see just how much of a speedup you’d see on 1/2 million rows using a combined index versus individual ones. (You should still not use columns col3 and col4 in the index, since you’re not joining anything by them.)

    • 0