feat(hogql): hidden aliases for special fields

[mariusandra]

Problem

The following doesn't work:

select * from (SELECT timestamp FROM events)

because we don't add AS timestamp in the generated ClickHouse SQL here when wrapping with a function:

SELECT timestamp FROM (SELECT toTimeZone(events.timestamp, %(hogql_val_0)s) FROM events WHERE equals(events.team_id, 1)) -- error

This also caused issues with the custom SQL columns I was trying to implement.

Changes

Now this works.

I wrapped each field with a "hidden alias" node, which will be ignored during printing... except if it's the first child of a select column.

Alternatives

This is not the first implementation of this, nor the only way to do it. An earlier proposal (from @Twixes) was effectively to use dedicated named columnExpr nodes for select fields, and only for select fields.

I thought about it hard, but in the end decided against it. A columnExpr is effectively just an alias that can be used in one place. It brings clarity in some places, but makes other things messier. I imagine we'd have to rework many places that build select queries from AST nodes. Then it'll also make copying nodes between trees harder, as you'll have to deal with remove_column_expr style functions everywhere.

All of this would have been doable, but we got most of it for free by reusing aliases... including correct printing, ignoring and type extraction (the ast.FieldAliasType was already there and integrated into a lot of places).

I hope this is the least evil best of both world approach, but time will tell. I'd personally be happy to merge it in and get the custom SQL column expressions live already :).

How did you test this code?

Wrote tests, adapted old ones, made sure it's all green.

[mariusandra]

This breaks some tests, so will convert to draft and finish Monday. 👋

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[mariusandra]

This is ready for a look! It's blocking a) custom SQL columns, b) PoEv2 with the overrides table, c) Lifecycle insights 😅

[Gilbert09]

Great test coverage