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Date: Sat, 10 May 2014 18:31:35 -0700
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Subject: Re: default opclass for jsonb (was Re: Call for
 GIST/GIN/SP-GIST opclass documentation)
From: Peter Geoghegan <pg@heroku.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Heikki Linnakangas <hlinnakangas@vmware.com>, Greg Stark <stark@mit.edu>,
 Bruce Momjian <bruce@momjian.us>,
 Gavin Flower <GavinFlower@archidevsys.co.nz>,
 "David E. Wheeler" <david@justatheory.com>,
 Robert Haas <robertmhaas@gmail.com>,
 Andrew Dunstan <andrew@dunslane.net>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
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On Sat, May 10, 2014 at 5:54 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I'm not sure whether we have consensus to rename jsonb_hash_ops to
> jsonb_path_ops, but since time is so short I went ahead and made a draft
> patch to do so (attached).  Probably the most interesting part of this is
> the new text in json.sgml explaining the difference between the two
> opclasses.  I also added a paragraph about the empty-query hazard that
> Peter mentions.  Do people think this discussion is correct and useful?

I for one am fine with the name change you propose.

> +     especially if
> +     there are a very large number of rows containing any single one of the
> +     three keys

I suggest that you phrase this as "three index items".

> +     A disadvantage of the <literal>jsonb_path_ops</literal> approach is
> +     that it produces no index entries for JSON structures not containing
> +     any values, such as <literal>{"a": {}}</literal>.  If a search for

I suggest "any values or elements".

Even though I previously called hashing an implementation detail, we
are bound to have to mention it in passing when discussing the
limitations of jsonb_hash_ops/jsonb_path_ops. I think that you should
proceed with committing the entire patch, including the doc changes
that discuss implementation details around the two GIN opclasses.

-- 
Peter Geoghegan


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