Optimizing Top K in Postgres

94 points - last Sunday at 10:54 PM

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bob1029 today at 8:05 AM
Lucene really does feel like magic sometimes. It was designed expressly to solve the top K problem at hyper scale. It's incredibly mature technology. You can go from zero to a billion documents without thinking too much about anything other than the amount of mass storage you have available.

Every time I've used Lucene I have combined it with a SQL provider. It's not necessarily about one or the other. The FTS facilities within the various SQL providers are convenient, but not as capable by comparison. I don't think mixing these into the same thing makes sense. They are two very different animals that are better joined by way of the document ids.

jmgimeno today at 7:16 AM
Maybe I'm wrong, but for this query:

SELECT * FROM benchmark_logs WHERE severity < 3 ORDER BY timestamp DESC LIMIT 10;

this index

CREATE INDEX ON benchmark_logs (severity, timestamp);

cannot be used as proposed: "Postgres can jump directly to the portion of the tree matching severity < 3 and then walk the timestamps in descending order to get the top K rows."

Postgres with this index can walk to a part of the tree with severity < 3, but timestamps are sorted only for the same severity.

davidelettieri today at 6:35 AM
The "But Wait, We Need Filters Too" paragraph mentions "US" filter which is introduced only later on.
h1fra today at 9:41 AM
Postgres is really good at a lot of things, but it's very unfortunate that it's really bad at simple analytics. I wish there was a plugin instead of having to have N databases
Vadim_samokhin today at 8:31 AM
Just in case, there is a btree_gin extension which can be used in queries combining gin-indexable column and btree-indexable column. It doesn’t solve top-K ordering problem though.
JEONSEWON today at 5:51 AM
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bbshfishe today at 6:50 AM
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tacone today at 7:24 AM
The issue here is the row based format. You simply can't filter on arbitrary columns with that. Either use an external warehouse or a columnar plug-in like Timescale.