Question

I’m querying my database (Postgres 8.4) with something like the following:

SELECT COUNT(*) FROM table WHERE indexed_varchar LIKE 'bar%';

The complexity of this is O(N) because Postgres has to count each row. Postgres 9.2 has index-only scans, but upgrading isn’t an option, unfortunately.

However, getting an exact count of rows seems like overkill, because I only need to know which of the following three cases is true:

• Query returns no rows.
• Query returns one row.
• Query returns two or more rows.

So I don’t need to know that the query returns 10,421 rows, just that it returns more than two.

I know how to handle the first two cases:

SELECT EXISTS (SELECT COUNT(*) FROM table WHERE indexed_varchar LIKE 'bar%');

Which will return true if one or more rows exists and false is none exist.

Any ideas on how to expand this to encompass all three cases in a efficient manner?

Answer 1

SELECT COUNT(*) FROM (
  SELECT * FROM table WHERE indexed_varchar LIKE 'bar%' LIMIT 2
) t;