Hello, Alexander! Please consider changes to the patch:

вт, 17 июл. 2018 г. в 16:59, Alexander Turenko <alexander.turenko@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).

Fixed:

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

Also fixed.