[tarantool-patches] Re: [PATCH] sql: assertion fail on nested select

[Alexander Turenko]

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).

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