Re: Proposal: "Causal reads" mode for load balancing reads without stale data

On 16 November 2015 at 18:44, Simon Riggs <simon@2ndquadrant.com> wrote:

The pooler knows which statements are reads and writes

I think that's an iffy assumption.

It's one we tend to make because otherwise read/write pooling won't work, but in PostgreSQL there's really no way to know when calling a function.

What does

    SELECT get_user_stats()

do? The pooler has _no_ _idea_ unless manually configured with knowledge about particular user defined functions.

In the absence of such knowledge it can:

- send the work to a replica and report the ERROR to the user if it fails due to an attempted write;

- send the work to a replica, capture an ERROR due to attempted write, and retry on the master;

- send everything it's not sure about to the master

If a pooler had insight into the catalogs and if we had readonly / readwrite attributes on functions, it could be smarter.