Follow list chaos and destruction

[alexgleason]

I am storing Nostr events in an SQLite database. Inserting a follow list can take more than 1 second due to indexing all the tags.

I created a benchmark to reproduce the behavior:

cpu: AMD Ryzen 7 PRO 5850U with Radeon Graphics
runtime: deno 1.40.2 (x86_64-unknown-linux-gnu)

file:///home/alex/Projects/NSpec/src/NDatabase.bench.ts
benchmark                           time (avg)        iter/s             (min … max)       p75       p99      p995
------------------------------------------------------------------------------------ -----------------------------
NDatabase.event                    258.35 µs/iter       3,870.8   (177.46 µs … 1.59 ms) 283.02 µs 544.9 µs 616.11 µs
NDatabase.event with many tags       1.98 ms/iter         505.2      (1.63 ms … 4.7 ms) 2 ms 3.82 ms 4.51 ms

It is over 7x slower to insert an event with 300 tags vs 1 tag.

• I need to index all p tags in follow lists, because I need the ability to check following relationships between any two users at any time.
• People update their follow list all the time. When they do, it freezes my backend service for all users.
• I've tried all the tricks: use WAL mode, set pragma synchronous = normal, experimenting with different ways of using transactions and formatting the query. Nothing makes the performance acceptable. Maybe I will try creating a worker pool so slow queries don't block other queries, but having 1s queries is still bad.

There is maybe some way of storing diffs so I don't have to insert so many tags for each event. This is related to #349 and #1036. I'm not sure the best way to handle this yet.

[vitorpamplona]

more than 1 second

1 millisecond, right?

1 Millisecond for 300 rows sounds about right.

[alexgleason]

On my powerful laptop the issue is barely noticeable. But on a commodity server (where I expect most users to run my software), it indeed takes more than 1 second:

Mar 05 20:29:18 gleasonator deno[1105112]:   ditto:sqlite.worker insert into "tags" ("event_id", "tag", "value") values (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), (?, ?, ?), 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(1.12s)

At the very end of that log:

(1.12s)

I'm using a dedicated worker thread for SQLite, so only queries can block other queries. I'm measuring it here.

EDIT: There are actually 967 entries in that particular follow list.

[vitorpamplona]

You know, this is exactly why we had DBAs back in my days. :)

[alexgleason]

That's just ChatGPT now. But it doesn't have a good answer for this one. 😂

[vitorpamplona]

The answer was usually: Switch to Oracle with two mirrored instances, one for writes and one for reads, and buy a faster server. :)

[vitorpamplona]

I have come to realize that the tag key is irrelevant for the tag's database index. The value is what matters.

Most of the time, if not all the time, we don't have the same value for separate keys. An event_id will not be on a p and e at the same time and if they are, there is always a need for additional filtration after a Nostr filter runs. The client can get all the values on something and then remove the invalid keys out later.

[alexgleason]

Btw, this is an in-memory SQLite. So the FS is not a bottleneck in any of these tests. It would probably be much worse on disk.

[vitorpamplona]

Very interesting.

Now I am starting to wonder if you can just do a trigger to insert all rows in the tags table.

Something like:

CREATE TRIGGER IF NOT EXISTS insert_tags AFTER INSERT ON nostr_events
BEGIN
  INSERT INTO nostr_tags (event_id, name, value)
  SELECT NEW.id,
         json_extract(t.value, '$[0]'),
         json_extract(t.value, '$[1]')
  FROM json_each(NEW.tags) AS t;
END;

I am not sure if it increases speed, but it should significantly reduce latency.

[alexgleason]

Performance of that is a ~60% increase!

benchmark                           time (avg)        iter/s             (min … max)       p75       p99      p995
------------------------------------------------------------------------------------ -----------------------------
NDatabase.event                    286.93 µs/iter       3,485.2    (204.35 µs … 3.6 ms) 302.02 µs 629.96 µs 759.6 µs
NDatabase.event with many tags       1.18 ms/iter         849.2     (1.03 ms … 3.39 ms) 1.14 ms 2.42 ms 2.46 ms

We lose some flexibility about which tags to index. But I think this shows we're on the right track. 😃

[vitorpamplona]

BTW, you can also create an index directly in the JSON fiends. No second table is needed.

https://antonz.org/json-virtual-columns/

[vitorpamplona]

Scratch that. I think the array breaks this idea (you would need a separate virtual table/view regardless)