[tarantool-patches] Re: [PATCH] sql: assertion fail on nested select
hollow653 at gmail.com
Wed Jul 18 15:34:42 MSK 2018
Hello, Alexander! Please consider changes to the patch:
вт, 17 июл. 2018 г. в 16:59, Alexander Turenko <
alexander.turenko at tarantool.org>:
> Hi Nikita!
> Please, consider comments below.
> WBR, Alexander Turenko.
> On Mon, Jul 02, 2018 at 03:11:40PM +0300, N.Tatunov wrote:
> > To optimize the select-subquery tarantool uses
> > subquery flattening function which is only used
> > with respect to some restrictions. One of them
> > was implemented improperly.
> Be certain: 'some restriction' and just 'improperly' are not good
> problem descriptions.
> I would use wording like the following.
> > When a subquery is a compound select the subquery flattening
> > optimization verify whether the ORDER BY clause presents, but only for
> > the last subquery component. The commit fixes that to verify all
> > subquery components.
> > In case of ORDER BY in non-last component of a compound select the
> > error should be raised. Accepting of such subqueries for the
> > flattening optimization prevents the error to be raised afterwards.
> > Instead it stuck into assertion violation inside flattenSubquery (see
> > the issue).
To optimize the select-subquery tarantool uses
subquery flattening optimization which is only used
with respect to some restrictions. When any of
subselects but the last one has an ORDER BY
clause we should raise an error. So subquery
components containing it are not accepted
to be optimized.
Checking part was only checking if the last
subselect contains the clause which led to an
assertion fail. With the patch applied
flattening is not used in case any of
subselects has an ORDER BY.
> I checked sqlite3 check-in 8000d230 (it is immediate predecessor of
> c9104b59 where the flattening optimization seems to be changed, but
> tarantool didn't updated with that check-in). It has the assert, but has
> no a change like yours, and correctly shows the following error:
> Error: ORDER BY clause should come after UNION ALL not before
> Why it is so?
> > +test:do_catchsql_test(
> > + "subquery-9.3",
> > + [[
> > + SELECT * FROM (SELECT * FROM table1 ORDER BY 1 UNION All
> > + SELECT * FROM table1 UNION All
> > + SELECT * FROM table1);
> All -> ALL
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