如果where子句的cols大于2，Postgres是否使用2-col索引？

Question

Suppose I have an index on 2 columns in my table.

Index on: title and subdomain.

If my query is

SELECT * FROM websites
where title = 'This is a title'
and subdomain = 'www.aol.com'
and meta_description = 'this is a meta desc'
and num_hits = 2

Will the 2-column index on title and subdomain still be used?

Answer 1

It may. the choice of when to use an index is a complicated one. PostgreSQL can use a multi-column index in this case. Whether it will depends on the size of the table, the selectivity of the first column in the index, and the like.

If you are asking whether PostgreSQL will rule out such an index, no it won't. It may even dynamically combine it with other indexes if it seems like a good idea.