PostgreSQL: speed up SELECT query in table with millions of rows

Question

I have a table with > 4.5 million rows and my SELECT query is far too slow for my needs.

The table is created with:

CREATE TABLE all_leg         


        

Answer 1

The index does not help because you use

WHERE dept_dt::date=constant

This seems fine to a beginner, but to the database, it looks like:

WHERE convert_timestamp_to_date(dep_ts)=constant

With convert_timestamp_to_date() being an arbitrary function (I just came up with the name, don't look it up in the docs). In order to use the index on dep_ts, the DB would have to reverse the function convert_timestamp_to_date into something like convert_date_to_timestamp_range (because a date corresponds to a range of timestamps, not just one timestamp), and then rewrite the WHERE as Laurenz did.

Since there are many such functions, the database developers didn't bother to maintain a huge table of how to invert them. Also it would only help for special cases. For example, if you specified a date range in your WHERE instead of a "=constant" then it would be yet another special case. It's your job to handle this ;)

Also, an index on (dep_dt,price_ct) won't speed up the sort as the first column is a timestamp, so the rows are not ordered in the index the way you want. You'd need an index on (dept_dt::date, price_ct) to eliminate the sort.

Now, which index to create? This depends...

If you also use timestamp range queries like "WHERE dep_dt BETWEEN ... AND ..." then the index on dep_dt needs to be the original timestamp type. In this case, creating another index on the same column, but converted to date, would be unnecessary (all indexes have to be updated on writes, so unnecessary indexes slow down inserts/updates). However, if you use the index on (dep_ts::date,price_ct) lots and lots of times and eliminating the sort is really important for you, then it may make sense. It's a tradeoff.