Re: [HACKERS] proposal: schema variables

On Thu, Oct 26, 2017 at 9:21 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:

Hi,

I propose a  new database object - a variable. The variable is persistent object, that holds unshared session based not transactional in memory value of any type. Like variables in any other languages. The persistence is required for possibility to do static checks, but can be limited to session - the variables can be temporal.

My proposal is related to session variables from Sybase, MSSQL or MySQL (based on prefix usage @ or @@), or package variables from Oracle (access is controlled by scope), or schema variables from DB2. Any design is coming from different sources, traditions and has some advantages or disadvantages. The base of my proposal is usage schema variables as session variables for stored procedures. It should to help to people who try to port complex projects to PostgreSQL from other databases.

The Sybase  (T-SQL) design is good for interactive work, but it is weak for usage in stored procedures - the static check is not possible. Is not possible to set some access rights on variables.

The ADA design (used on Oracle) based on scope is great, but our environment is not nested. And we should to support other PL than PLpgSQL more strongly.

There is not too much other possibilities - the variable that should be accessed from different PL, different procedures (in time) should to live somewhere over PL, and there is the schema only.

The variable can be created by CREATE statement:

CREATE VARIABLE public.myvar AS integer;

CREATE VARIABLE myschema.myvar AS mytype;

CREATE [TEMP] VARIABLE [IF NOT EXISTS] name AS type

  [ DEFAULT expression ] [[NOT] NULL]

  [ ON TRANSACTION END { RESET | DROP } ]

  [ { VOLATILE | STABLE } ];

It is dropped by command DROP VARIABLE  [ IF EXISTS] varname.

The access rights is controlled by usual access rights - by commands GRANT/REVOKE. The possible rights are: READ, WRITE

The variables can be modified by SQL command SET (this is taken from standard, and it natural)

SET varname = expression;

Unfortunately we use the SET command for different purpose. But I am thinking so we can solve it with few tricks. The first is moving our GUC to pg_catalog schema. We can control the strictness of SET command. In one variant, we can detect custom GUC and allow it, in another we can disallow a custom GUC and allow only schema variables. A new command LET can be alternative.

The variables should be used in queries implicitly (without JOIN)

SELECT varname;

The SEARCH_PATH is used, when varname is located. The variables can be used everywhere where query parameters are allowed.

I hope so this proposal is good enough and simple.

Comments, notes?

I have a question on this.  Since one can issue set commands on arbitrary settings (and later ALTER database/role/system on settings you have created in the current session) I am wondering how much overlap there is between a sort of extended GUC with custom settings and variables. 

Maybe it would be simpler to treat variables and GUC settings to be similar and see what can be done to extend GUC in this way?

I mean if instead we allowed restricting SET to known settings then we could have a CREATE SETTING command which would behave like this and then use SET the same way across both.

In essence I am wondering if this really needs to be as separate from GUC as you are proposing.

If done this way then:

1.  You could issue grant or revoke on GUC settings, allowing some users but not others to set things like work_mem for their queries

2.  You could specify allowed types in custom settings.

3.  In a subsequent stage you might be able to SELECT .... INTO setting_name FROM ....;  allowing access to setting writes based on queries.

regards

Pavel

--

Best Regards,

Chris Travers

Database Administrator

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