On 05/02/2017 07:00 PM, Tomas Vondra wrote:
> I think we agree that:
> * Just removing the optimization fence and telling users to use OFFSET
> 0 instead is a no-go, just like removing the fencing and not providing
> any sensible replacement.
> * GUC is not the solution.


> Which leaves us with either WITH INLINE or WITH MATERIALIZE, or
> something along those lines.
> If we go with WITH INLINE then we're likely not solving anything,
> because most people will simply use WITH just like now, and will be
> subject to the fencing without realizing it.

In many cases it won't actually matter much.

We're going to penalize some group of users no matter what we do. It
just seems a pity that it might be the group who actually took us at our
word. It's no skin off my nose - I will happily spend time finding
places where this will make things worse for customers.

> Or we will choose WITH MATERIALIZE, and then the users aware of the
> fencing (and using the CTEs for that purpose) will have to modify the
> queries. But does adding MATERIALIZE quality as major query rewrite?

It's not a major rewrite. But I can think of at least one former
customer who will have to go through a heck of a lot of code finding
where to add that one word.

> Perhaps combining this with a GUC would be a solution. I mean, a GUC
> specifying the default behavior, and then INLINE / MATERIALIZE for
> individual CTEs in a query?
> If you have an application intentionally using CTEs as a fence, just do
>     ALTER DATABASE x SET enable_guc_fencing = on
> and you don't have to rewrite the queries.

ITYM enable_cte_fencing.

I'm not sure it will help all that much unless we provide both decorator
variants, so people can remediate their code one query at a time, and
this guc would just govern the default.



Andrew Dunstan                https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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