Changes for PostgreSQL compatibility

[jseldess]

• PR: sql: add alias timezone for time zone session var cockroach#19931
  • The time zone session variable (with a space) has been renamed timezone (without a space), and SET TIMEZONE and SHOW TIMEZONE are now supported alongside the existing SET TIME ZONE and SHOW TIME ZONE syntax. Also, SET TIMEZONE = can now be used as an alternative to SET TIMEZONE TO.
• PR: sql: transaction_read_only should return off cockroach#19971
  • The transaction_read_only session variable is now supported. It is always set to off.
• PR: sql: name the transaction settings like PG does cockroach#20264
  • The transaction isolation level, transaction priority, and transaction status session variables have been renamed transaction_isolation, transaction_priority, and transaction_status.

[jseldess]

Need to update SET (session variable), SHOW (session variable), and SET TRANSACTION, where relevant.

It looks like we don't list some of the transaction variables on the set/show variables pages. Not sure why not.

[lnhsingh]

It seems like you can't interchangeably use TIME ZONE and TIMEZONE. @knz, can you help clarify? For example, if I SET TIME ZONE DEFAULT and then SET TIMEZONE EST, the second SET doesn't work unless I use SET TIMEZONE = EST. Why is this?

[lnhsingh]

Also: The transaction_read_only can't be changed from off, so it seems misleading that you can SET transaction_read_only = FALSE / TRUE; and get a SET (even though nothing is changed). @justinj, is this just a wonky or should this be changed?

[justinj]

Hm, yes, I think I assumed in that PR that attempting to set it would fail, we should fix that. Opened as cockroachdb/cockroach#24608

[knz]

@lhirata: SET TIME ZONE xxx is a "special form", which has its own, special cased grammar rule, and gets automagically replaced to SET timezone = xxx.

This is similar to SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL xxx which is a special form that gets automagically replaced to SET TRANSACTION ISOLATION LEVEL xxx

We already mention in the text it's a special form. The only thing that needs to change in the doc page is the name of the variable in the table, and further references to the variable in that page and other pages.

[knz]

In the current text we say "SET TIME ZONE xxx is a special form for SET "time zone" = xxx". This text needs to be changed to "SET TIME ZONE xxx is a special form for SET timezone = xxx".

[lnhsingh]

@knz thanks for the explanation!