Ethan
c4db03f11a
## Summary
- add an `IS DISTINCT FROM` guard to `InsertChatMessage`'s
`updated_chat` CTE so `chats.last_model_config_id` is only rewritten
when the incoming `model_config_id` actually changes
- regenerate the query layer
- add focused regression coverage for the two meaningful behaviors:
same-model inserts and real model switches
- trim redundant message-field assertions so the new test stays focused
on the guard behavior
## Proof this is an improvement
This PR reduces work in the hottest chat write query without changing
the insert behavior.
### Why the old query did unnecessary work
Before this change, `InsertChatMessage` always ran this update whenever
`model_config_id` was non-null:
```sql
UPDATE chats
SET last_model_config_id = sqlc.narg('model_config_id')::uuid
WHERE id = @chat_id::uuid
AND sqlc.narg('model_config_id')::uuid IS NOT NULL
```
That means the query rewrote the `chats` row even when
`chats.last_model_config_id` was already equal to the incoming value.
### What changes in this PR
This PR adds:
```sql
AND chats.last_model_config_id IS DISTINCT FROM sqlc.narg('model_config_id')::uuid
```
So same-model inserts still insert the message, but they no longer
perform a redundant `UPDATE chats`.
### Why this matters on the hot path
From the chat scaletest investigation that motivated this change:
- `InsertChatMessage` (+ `updated_chat` CTE) was the hottest write query
- about **104k calls**
- about **0.69 ms average latency**
- about **71.8 s total DB execution time**
We also verified common callsites where the update is provably
redundant:
- `CreateChat` inserts the chat with `LastModelConfigID =
opts.ModelConfigID`, then immediately inserts initial system/user
messages with that same model config
- follow-up user messages commonly pass `lockedChat.LastModelConfigID`
straight into `InsertChatMessage`
- assistant/tool/summary persistence keeps the current model in the
common case; only real switches or fallback cases need the chat row
update
That means a meaningful fraction of executions of the hottest DB write
query move from:
- **before:** insert message **+** rewrite chat row
- **after:** insert message only
This should reduce row churn and write contention on `chats`, especially
against other chat-row writers like `UpdateChatStatus` and
`GetChatByIDForUpdate`.
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