precomputing user-defined functions in postgresql

Question

I'm trying to precompute a user-defined function on a per row basis. The idea is I have JSON object as a text object in one of the fields, and I want to parse out some other 'fields' from it, which can be returned in queries just like any other true field. However, the overhead of parsing the JSON is significant. Is there any way to precompute this parsing function in a way that speeds up queries?

Please refrain from arguing that there shouldn't be JSON as text on the database in the first place; I am aware of the pros and cons.

Answer 1

First off, you may be interested in the upcoming JSON data type of PostgreSQL 9.2 (to be released soon, now).

As to your question, you are looking for a materialized view (or the simpler form: a redundant precomputed column in your table). "Materialized View" is just the established term, not a special object in a PostgreSQL database. Basically you create a redundant table with precomputed values, that you refresh at certain events or on a timely basis.

A search for the term will give you some answers.

Answer 2

In addition to a materialized view, keep in mind that PostgreSQL can also index functions' output so you can do something like:

CREATE INDEX my_foo_bar_udf_idx ON foo (bar(baz));

This works only if the UDF is marked as immutable meaning output only depends on arguments. This gives you an option to run your function against the query arguments and then scan the index instead of the table. It doesn't meet all use cases, but it does meet many of them and it can often save you the headaches of materializing views.