PostgreSQL: Create index for boolean column

Question

I have a table that has one boolean column.

productid integer
isactive boolean

When I execute the query

SELECT productid           


        

Answer 1

PostgreSQL will use an index only if it thinks it will be cheaper that way. An index on a boolean column, which can only take two possible values, will almost never be used, because it is cheaper to sequentially read the whole table than to use random I/O on the index and the table if a high percantage of the table has to be retrieved.

An index on a boolean column is only useful

1. in data warehouse scenarios, where it can be combined with other indexes via a bitmap index scan.

2. if only a small fraction of the table has the value TRUE (or FALSE for that matter). In this case it is best to create a partial index like

   CREATE INDEX ON mytab((1)) WHERE boolcolumn;