Re: Schema variables - new implementation for Postgres 15

New organization

1. basic CREATE VARIABLE, DROP VARIABLE, GRANT, REVOKE, ALTER, pg_dump

2. basic SELECT var, LET var = value

3. DISCARD VARIABLES

4. cleaning memory used by dropped variables

5. temporary variables + ON COMMIT DROP clause support

6. ON TRANSACTION END RESET clause support

7. DEFAULT expr clause support

8. support NOT NULL and IMMUTABLE clauses

9. use message "column or variable doesn't exists" instead "column doesn't exists"

Regards

Pavel