Hi
I was asked to write the most reduced implementation of session variables. I thought about possible reductions and there is one - catalog. We
can implement temporary session variables. In this case the catalog is not necessary. After reduction the total size of the patchset is 134KB (doc + code + tests). The size of the previous patchset was 390KB.
Unfortunately - without catalog some the dependency check is not available, and then features that depends on dependencies are not available:
1. no catalog, no dependencies - only buildin types can be used (no custom types, no domains),
2. no catalog, no dependencies - variables cannot be used in objects that depend on some objects (views, sql functions),
3. simplified syntax - only scalar types can be used (no arrays, no composites),
4. no catalog, no access rights, only owner check (variables can be used just by their creator (owner)),
5. no catalog, no plan cache invalidation support (instead type check before any usage),
6. no catalog, only temp variables are supported - schema cannot be specified,
7. without direct access from the expression executor - variables cannot be a parameter of CALL statement
8. session variables block parallel execution
9. no catalog, no object address - no event triggers for session variables
10. no catalog - DDL for session variables (CREATE TEMP VARIABLE, DROP VARIABLE) are not transactional. Without catalog, implementation of transactional DDL can be very difficult.
Although there are lot of strong limits, I think so implemented feature can be still useful (parametrization of anonymous block and secure session scope storage)
CREATE TEMP VARIABLE x AS int;
CREATE TEMP VARIABLE y AS int;
LET x = 100;
SELECT VARIABLE(x);
DO $$
BEGIN
RAISE NOTICE '%', VARIABLE(x);
LET y = VARIABLE(x) + 10;
END $$;
SELECT VARIABLE(y);
DROP VARIABLE x;
DROP VARIABLE y;
This simple and reduced implementation doesn't block continuous development in different directions (without compatibility breaks).
When I worked on this reduced version I found the importance of catalog based implementation. It is almost impossible to implement variables with life scope longer than transactions without a catalog. There is a strong necessity of using dependency mechanisms. With catalog, the plan cache invalidation can be used (instead typecheck). It is more robust.
Comments, notes?
Regards
Pavel
