From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruce Momjian <bruce@momjian.us>
cc: "David E. Wheeler" <david@justatheory.com>, Greg Stark <stark@mit.edu>,
 Robert Haas <robertmhaas@gmail.com>,
 Heikki Linnakangas <hlinnakangas@vmware.com>,
 Andrew Dunstan <andrew@dunslane.net>, Peter Geoghegan <pg@heroku.com>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: default opclass for jsonb (was Re: Call for GIST/GIN/SP-GIST
 opclass documentation)
In-reply-to: <20140508134701.GO30817@momjian.us>
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Comments: In-reply-to Bruce Momjian <bruce@momjian.us>
 message dated "Thu, 08 May 2014 09:47:01 -0400"
Date: Thu, 08 May 2014 10:16:54 -0400
Message-ID: <5819.1399558614@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

Bruce Momjian <bruce@momjian.us> writes:
> On Tue, May  6, 2014 at 06:20:53PM -0400, Tom Lane wrote:
>> I wonder why there's not an option to store keys and values separately,
>> but as hashes not as the original strings, so that indexability of
>> everything could be guaranteed.  Or a variant of that might be to hash
>> only strings that are too large to fit in an index entry, and force
>> recheck only when searching for a string that needed hashing.
>> 
>> I wonder whether the most effective use of time at this point
>> wouldn't be to fix jsonb_ops to do that, rather than arguing about
>> what to rename it to.  If it didn't have the failure-for-long-strings
>> problem I doubt anybody would be unhappy about making it the default.

> Can we hash just the values, not the keys, in jsonb_ops, and hash the
> combo in jsonb_hash_ops.  That would give us key-only lookups without a
> recheck.

No, because there's nothing in JSON limiting the length of keys, any more
than values.

I think the idea of hashing only keys/values that are "too long" is a
reasonable compromise.  I've not finished coding it (because I keep
getting distracted by other problems in the code :-() but it does not
look to be very difficult.  I'm envisioning the cutoff as being something
like 128 bytes; in practice that would mean that few if any keys get
hashed, I think.

			regards, tom lane


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