Increase Element model varchar limits

[Twixes]

Changes

Model field change to support larger elements in Postgres PostHog and in ClickHouse webhooks (which do matching in Postgres). Resolves #3913.

[paolodamico]

lgtm!

I believe CharField & TextField have no significant differences with a Postgres backend, so just a nit, but maybe TextField would better reflect these attributes?

[macobo]

This operation WILL cause issues both in prod and for self-hosted.

ALTER TABLE on huge tables is insanely expensive since it needs to rewrite the whole table.

[Twixes]

@paolodamico
We could, but I think there is a size we should not cooperate with, so I'd leave it some limit in. Ridiculously big elements could cause performance problems.

@macobo
This is not the case for this type of operation since Postgres 9.2 @macobo.
From https://www.postgresql.org/docs/9.2/release-9-2.html#AEN114949:

Increasing the length limit for a varchar or varbit column, or removing the limit altogether, no longer requires a table rewrite.

[macobo]

You're correct - these should be instantaneous assuming no long-running queries holding locks block them.

posthog> select count(*) from posthog_element;
+----------+
| count    |
|----------|
| 26525865 |
+----------+
posthog> ALTER TABLE posthog_element ALTER COLUMN text TYPE VARCHAR(1000);
ALTER TABLE
Time: 0.166s