Create a unique index on a non-unique column

Question

Not sure if this is possible in PostgreSQL 9.3+, but I\'d like to create a unique index on a non-unique column. For a table like:

CREATE TABLE data (
  id SE         


        

Answer 1

An index can only index actual rows, not aggregated rows. So, yes, as far as the desired index goes, creating a table with unique values like you mentioned is your only option. Enforce referential integrity with a foreign key constraint from data.day to days.day. This might also be best for performance, depending on the complete situation.

However, since this is about performance, there is an alternative solution: you can use a recursive CTE to emulate a loose index scan:

WITH RECURSIVE cte AS (
   (  -- parentheses required
   SELECT day FROM data ORDER BY 1 LIMIT 1
   )
   UNION ALL
   SELECT (SELECT day FROM data WHERE day > c.day ORDER BY 1 LIMIT 1)
   FROM   cte  c
   WHERE  c.day IS NOT NULL  -- exit condition
   )
SELECT day FROM cte;

Parentheses around the first SELECT are required because of the attached ORDER BY and LIMIT clauses. See:

• Combining 3 SELECT statements to output 1 table

This only needs a plain index on day.

There are various variants, depending on your actual queries:

• Optimize GROUP BY query to retrieve latest row per user
• Unused index in range of dates query
• Select first row in each GROUP BY group?

More in my answer to your follow-up querstion:

• Counting distinct rows using recursive cte over non-distinct index