Re: default opclass for jsonb (was Re: Call for GIST/GIN/SP-GIST opclass documentation)

I wrote:
> 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.

Attached is a draft patch for this. In addition to the hash logic per se,
I made these changes:

* Replaced the K/V prefix bytes with a code that distinguishes the types
of JSON values. While this is not of any huge significance for the
current index search operators, it's basically free to store the info,
so I think we should do it for possible future use.

* Fixed the problem with "exists" returning rows it shouldn't. I
concluded that the best fix is just to force recheck for exists, which
allows considerable simplification in the consistent functions.

* Tried to improve the comments in jsonb_gin.c.

Barring objections I'll commit this tomorrow, and also try to improve the
user-facing documentation about the jsonb opclasses.

regards, tom lane