Re: TABLESAMPLE patch

On 9 April 2015 at 04:12, Michael Paquier <michael.paquier@gmail.com> wrote:

> Also, I am wondering if the sampling logic based on block analysis is
> actually correct, for example for now this fails and I think that we
> should support it:
> =# with query_select as (select generate_series(1, 10) as a) select
> query_select.a from query_select tablesample system (100.0/11)
> REPEATABLE (9999);
> ERROR:  42P01: relation "query_select" does not exist
>
> How does the SQL spec define exactly TABLESAMPLE? Shouldn't we get a
> sample from a result set?
> Thoughts?

Good catch. The above query doesn't make any sense.

TABLESAMPLE SYSTEM implies system-defined sampling mechanism, which is
block level sampling. So any block level sampling method should be
barred from operating on a result set in this way... i.e. should
generate an "ERROR inappropriate sampling method specified"

TABLESAMPLE BERNOULLI could work in this case, or any other non-block
based sampling mechanism. Whether it does work yet is another matter.

This query should be part of the test suite and should generate a
useful message or work correctly.

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