Since you select a couple of big columns an index-only scan is probably not a viable option.
This code works (if no NULL values in data!)
While the column isn't defined
NOT NULL, add
NULLS LAST to the sort order to make it work in any case, even with
NULL values. Ideally, use the clause in the corresponding index as well:
SELECT <some big columns>
ORDER BY when_ DESC NULLS LAST
Without any index on
when_ column, does this statement require a full
scan of all rows?
Yes. Without index, there is no other option left. (Well, there is also table partitioning where an index on key columns(s) is not strictly required, and it could assist with partition pruning. But you would typically have an index on key columns there, too.)
With an index on
when_ column, should I change this SQL to use some
other approach/strategy of query?
Basically, this is the perfect query. There are options in combination with advanced indexing:
NOT NULL column. Else, add
NULLS LAST to index and queries as suggested above.
You have a constant influx of rows with later
when_. Assuming the latest
_when constantly increases and never (or rarely) decreases (latest rows deleted / updated), you can use a very small partial index.
Run your query once to retrieve the latest
when_, subtract a safe margin (to be safe against losing the latest rows) and create an
IMMUTABLE function based on it. Basically a "fake global constant":
CREATE OR REPLACE FUNCTION f_when_cutoff()
RETURNS timestamptz LANGUAGE sql COST 1 IMMUTABLE PARALLEL SAFE AS
$$SELECT timestamptz '2015-07-25 01:00+02'$$;
PARALLEL SAFE only in Postgres 9.6 or later.
Create a partial index excluding older rows:
CREATE INDEX my_table_when_idx ON my_table_ (when_ DESC)
WHERE when_ > f_when_cutoff();
With millions of rows, the difference in size can be dramatic. And this only makes sense with a much smaller index. Just half the size or something would not cut it. Index access itself is not slowed much by a bigger index. It's mostly the sheer size of the index, which needs to be read and cached. (And possibly avoiding additional index writes, but hardly in your case.)
Use the function in all related queries. Include the same
WHERE condition (even if logically redundant) to convince the query planner the index is applicable. For the simple query:
SELECT <some big columns>
WHERE when_ > f_when_cutoff()
ORDER BY when_ DESC
The size of the index grows with new (later) entries. Recreate the function with a later timestamp and
REINDEX from time to time with no or little concurrent access. Only reindex after a relevant number of rows has been added. A couple of thousand entries won't matter much. We are doing this to cut off millions.
The beauty of it: queries don't change.
Implementation with function to update the partial index automatically:
More general advice: