Handle dump/restore/upgrade, take 2

[adamguo0]

(follow up on #128, re #118)

This is a second attempt at supporting pg_dump/pg_restore/pg_upgrade for this extension. This PR makes bigger design changes in order to provide a seamless dump/restore/upgrade experience. Users don't have to run any pre-dump or post-restore scripts in order to get their IMMVs working.

Summary of changes:

• Replace the viewdef column with a querystring column that stores the fully-qualified query string of each IMMV.
• Add a immvuuid column of uuid type. This is used as a unique identifier for IMMVs instead of the immvrelid.
• Set the IMMV triggers to external instead of internal so that they are automatically dumped by pg_dump. Set the trigger argument to immvuuid instead of immvrelid and let the triggers look up the corresponding OID of the IMMV.
• Create two C event triggers to keep the querystring up to date if any objects are renamed:
  • Before any ALTER RENAME statement, parse all IMMV query strings and store the Query nodes in a hash table.
  • After any ALTER RENAME statement, restore all Query nodes from the hash table back into pg_ivm_immv.

When users run pg_dump, the dump file contains the pg_ivm_immv table with the correct fully-qualified query strings, as well as the CREATE TRIGGER commands to re-create the IMMV triggers. The triggers use the immvuuid argument to look up the corresponding IMMV in pg_ivm_immv. The same goes for pg_upgrade. I ran some basic tests and it works so far with no user intervention needed.

Some todos:

• Handle upgrading from 1.10 to 1.11 given the significant implementation changes.
• Handle possible UUID collision (albeit very improbable).
• Come up with more precise criteria for when to re-parse the query strings (instead of re-parsing on every ALTER RENAME statement).

Eager for any feedback, thanks!

[michaelpq]

This patch set is very disorganized. Could it be possible to get a cleaned up branch with one or more commits and dedicated commit messages to understand a bit better what you are trying to achieve here? You could force push things, once the patch has been cleaned up, but it is not in a reviewable state as it stands.