It is kind of a logic fix in that you're doing less work by disregarding the sentinel value of 0. This has two improvements:
Avoiding expanding the chains dictionary unnecessarily.
Avoiding passing more arguments through the SQL engine (and to the database) which we know will result in no matches.
Long version:
In `_get_auth_chain_ids_using_cover_index_txn()`:
https://github.com/matrix-org/synapse/blob/c14a7de6af0b8a3cbf2e17afca1cab339bc5912d/synapse/storage/databases/main/event_federation.py#L300-L304
Seems to have a small logic problem, where the `max()` can evaluate to `0`. The `chains` dict is passed into the following SQL block:
https://github.com/matrix-org/synapse/blob/c14a7de6af0b8a3cbf2e17afca1cab339bc5912d/synapse/storage/databases/main/event_federation.py#L314-L325
where it inhabits the `max_seq` variable. Inside the `event_auth_chains` database table, that column can never be `0`.
Filter out that `0` before it gets to the SQL.
The temporary metric attached produces results that appear like this(over less than 24 hours):
The metric commit isn't particularly efficient and has no long term relevance, so it will not be included in the final render.
Short version:
Long version:
In `_get_auth_chain_ids_using_cover_index_txn()`: https://github.com/matrix-org/synapse/blob/c14a7de6af0b8a3cbf2e17afca1cab339bc5912d/synapse/storage/databases/main/event_federation.py#L300-L304 Seems to have a small logic problem, where the `max()` can evaluate to `0`. The `chains` dict is passed into the following SQL block: https://github.com/matrix-org/synapse/blob/c14a7de6af0b8a3cbf2e17afca1cab339bc5912d/synapse/storage/databases/main/event_federation.py#L314-L325 where it inhabits the `max_seq` variable. Inside the `event_auth_chains` database table, that column can never be `0`. Filter out that `0` before it gets to the SQL. The temporary metric attached produces results that appear like this(over less than 24 hours):  The metric commit isn't particularly efficient and has no long term relevance, so it will not be included in the final render.Pull Request Checklist
Signed-off-by: Jason Little realtyem@gmail.com