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I'm building a chat application (Yes, another one :P) and am really curious about the best way to store a chat's history(String username, String Message, Time, and possibly string channel depending on how I end up setting it up.

Since I'm using PostgreSQL anyway, I was tempted for going with it for chat history. However, I don't see a sane approach for using it to do this without generating a new row for every message, something obviously not very scale-able.

I know some MongoDB and was thinking that using a json array for each channel and storing the history in that would work well.

Is using two entirely databases in the same application a terrible idea?

Are there any other alternatives I should take a look at? (I'm open to pretty much anything)

Is there a popular PostgreSQL way of doing this? I took a look at a JSON columns, but it seemed fairly new and limited at the moment.

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    "However, I don't see a sane approach for using it to do this without generating a new row for every message, something obviously not very scale-able" . Why not? So long as you use a commit delay or async commit, and/or batch messages into transactions, you should be fine. – Craig Ringer Mar 12 '14 at 4:47
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    Adding to @CraigRinger's comment, I am wondering if constantly updating the MongoDB document for a given chat is faster than storing rows in PostgreSQL (or fast enough for being a viable solution). – dezso Mar 12 '14 at 8:18
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    I'll add a warning - having an unbounded array is a very bad design approach for MongoDB - it causes the document to constantly grow (has a limit of 16MB) and can cause problems. It would be a better idea to have documents per message and collections for each channel, but likely testing would be needed to see how well/badly it would work. Hard to speak to best practice with such a general description. – Adam C Mar 12 '14 at 18:40
  • Thanks for the responses. I was concerned that the number of rows per table would get out of hand, as it would be easy for even a lightly used chat service to generate thousands of messages daily. It seems like I would run into the same problems using mongodb as well though. – joshua-anderson Mar 12 '14 at 23:00
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You can use hstore for use EAV. hstore is available from pgsql 8.4. If you have installed json you can then use function hstore_to_json(hstore)

select hstore_to_json('"a key"=>1, b=>t, c=>null, d=>12345, e=>012345, f=>1.234, g=>2.345e+4')

this will output:

{"a key": "1", "b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4"}

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