We could create an index like this:

CREATE INDEX news_cats ON newsitem_categories(newsid, categoryid);

However, doing this limits some ways the index can be used.

A query against that table that users both 'newsid' and 'categoryid' will be able to use this index.

A query against that table that only gets the 'newsid' will be able to use the index.

A query against that table that only gets the 'categoryid' will not be able to use the index.


For a table like this:

CREATE TABLE example (
a int,
b int,
c int
);

This index:
CREATE INDEX example_index ON example(a,b,c);

It will be used when you check against 'a'.

It will be used when you check against 'a' and 'b'.

It will be used when you check against 'a', 'b' and 'c'.

It will not be used if you check against 'b' and 'c', or if you only check 'b' or you only check 'c'.

It will be used when you check against 'a' and 'c' but only for the 'a' column - it won't be used to check the 'c' column as well.


A query against 'a' OR 'b' like this:
SELECT a,b,c FROM example where a=1 OR b=2;

will only be able to use the index to check the 'a' column as well - it won't be able to use it to check the 'b' column.

Multicolumn indexes have quite specific uses, so check their use carefully.