Re: [HACKERS] proposal: schema variables

2018-03-12 16:38 GMT+01:00 Pavel Luzanov <p(dot)luzanov(at)postgrespro(dot)ru>:

>
> On 12.03.2018 09:54, Pavel Stehule wrote:
>
>
> 2018-03-12 7:49 GMT+01:00 Pavel Luzanov <p(dot)luzanov(at)postgrespro(dot)ru>:
>
>>
>> Is there any chances that it will work on replicas?
>>
> ...
>
> sure, it should to work. Now, I am try to solve a issues on concept level
> - the LET code is based on DML code base, so probably there is check for rw
> transactions. But it is useless for LET command.
>
>
> Very, very good!
>
> As I understand, the work on this patch now in progress and it not in
> commitfest.
> Please explain what features of schema variables I can review now.
>
> From first post of this thread the syntax of the CREATE VARIABLE command:
> CREATE [TEMP] VARIABLE [IF NOT EXISTS] name AS type
> [ DEFAULT expression ] [[NOT] NULL]
> [ ON TRANSACTION END { RESET | DROP } ]
> [ { VOLATILE | STABLE } ];
>

Now, it is too early for review - it is in development. Some features are
not implemented yet - DEFAULTs, ON TRANSACTION END .., others has not sense
(what I know now VOLATILE, STABLE). Schema variables are passed as
parameters to query, so the behave is like any other params - it is STABLE
only.

>
> But in psql I see only:
> \h create variable
> Command: CREATE VARIABLE
> Description: define a new permissioned typed schema variable
> Syntax:
> CREATE VARIABLE [ IF NOT EXISTS ] name [ AS ] data_type ]
>
> I can include DEFAULT clause in CREATE VARIABLE command, but the value not
> used:
> postgres=# create variable i int default 0;
> CREATE VARIABLE
> postgres=# select i;
> i
> ---
>
> (1 row)
>
> postgres=# \d+ i
> schema variable "public.i"
> Column | Type | Storage
> --------+---------+---------
> i | integer | plain
>
>
defaults are not implemented yet

>
> BTW, I found an error in handling of table aliases:
>
> postgres=# create variable x text;
> CREATE VARIABLE
> postgres=# select * from pg_class AS x where x.relname = 'x';
> ERROR: type text is not composite
>
> It thinks that x.relname is an attribute of x variable instead of an alias
> for pg_class table.
>
>
It is not well handled collision. This should be detected and prohibited.
In this case, because x is scalar, then x.xx has not sense, and then it
should not be handled like variable. So the current design is not too
practical - it generates more collisions than it is necessary and still,
there are some errors.

Now, there is one important question - storage - Postgres stores all
objects to files - only memory storage is not designed yet. This is part,
where I need a help.

Regards

Pavel

>
> -----
> Pavel Luzanov
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company
>
>