Disable PostgreSQL query optimization?

Question

I have come across the need to totally disable PostgreSQL query optimization just so that I can measure the effectiveness of applying QO (I want to see a before and after). I have come across this topic How to disable all optimizations of PostgreSQL from 2014 which basically says it's not possible. I wonder if anything has changed since then? I would really like to see a bad plan vs good plan scenario with a dramatic difference.

jjanes · Accepted Answer · 2020-03-25 19:53:22Z

3

You can't execute a query without a plan, and the optimizer is what comes up with the plan. There is no "default" plan which would exist in the absence of the optimizer.

You turn all the enable_* parameters to off, except for enable_seqscan and enable_nestloop.

Or you could set join_collapse_limit and from_collapse_limit both 1, that would severely constrain what the optimizer can do.

answered Mar 25, 2020 at 19:53

jjanes

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