Compressing large column by moving it to a unique table

[Adrian Garcia Badaracco <adrian@xxxxxxxxxxxx>]

I am using Timescale so I'll be mentioning some timestamp stuff but I think this is a general postgres question for the most part.

I have a table with some fixed, small columns (id, timestamp, etc) and a large JSONB column (let's call it `attributes`). `attributes` has 1000s of schemas, but given a schema, there's a lot of duplication. Across all rows, more than 99% of the data is duplicated (as measured by `count(attributes)` vs `count(distinct attributes)`.

I can't normalize `attributes` into real columns because it is quite variable (remember 1000s of schemas).

My best idea is to make a table like `(day timestamptz, hash text, attributes jsonb)` and then in my original table replace `attributes` with a reference to `new_table`. I can then make a view that joins them `select original_table.timestamp, new_table.attributes from original join new_table on (time_bucket('1 day', timestamp) = day AND original.hash = new_table.hash)` or something like that. The idea of time bucketing into 1 day is to balance write and read speed (by relying on timescale to do efficient time partitioning, data retention, etc.).

I recognize this is essentially creating a key-value store in postgres and also janky compression, so I am cautious about it.

Benchmarks showed huge wins (26x runtime, 100x buffers) for a selective query on `data`:

SELECT count(*)
FROM joined_view
WHERE (
  (attributes->'http.status_code')::int = 422
  AND
  start_timestamp > (now() - interval '2 day')
  AND
  start_timestamp < (now() - interval '1 day')
);

As expected `SELECT * FROM joined_view` is slower than `SELECT * FROM original`, but not by much (5x slower, 2x buffers, but they're both slow).

What I was sad to realize was that `SELECT start_timestamp FROM joined_view` is also slow because the query planner can't know that the join becomes a no-op if you don't select the `attributes` column (since there might be more than 1 match for each left row, thus modifying the result even if `attributes` wasn't selected). I don't know that this is a deal breaker but it's certainly a bummer.

Does anyone have any suggestions for this sort of situation?

Thank you,

Adrian