PostgreSQL: SELECT * from table WHERE timestamp IS WITHIN THIS MONTH - sql

Question

For some reason I'm kind of lost on how to archive:
SELECT * FROM table WHERE timestamp IS WITHIN THIS MONTH;
I've looked at https://www.postgresql.org/docs/9.4/static/functions-datetime.html, but are only able to select X days backwards.
I'm running PostgreSQL 9.4

Answer 1

... WHERE date_trunc('month', timestamp)
= date_trunc('month', current_timestamp);
Alternatively:
... WHERE timestamp >= date_trunc('month', current_timestamp)
AND timestamp < date_trunc('month', current_timestamp) + INTERVAL '1 month';
The second version can use an index on timestamp, the first would need one on the expression date_trunc('month', timestamp).

Answer 2

Why don't you just filter the month with between ?
Pass the start of this month as variable1, and the end of this month as variable2...
SELECT * FROM table WHERE
timestamp >= __month_start AND timestamp < __next_month_start
e.g.
SELECT * FROM table
WHERE
(
timestamp >= '20170701'::timestamp
AND
timestamp < '20170801'::timestamp
)
Unlike using functions in the where-clause, this maintains sargability.

Answer 3

What Laurenz Albe suggested will work, however you're going to have a performance penalty because you'll lose cardinality on that field, you either have to index expression you're going to query (Apparently PostgreSQL allows to do that: https://www.postgresql.org/docs/current/static/indexes-expressional.html) or create a separate column to store yyyy-mm values and query it.