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From: Nikita Glukhov <n.gluhov@postgrespro.ru>
To: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Stas Kelvich <s.kelvich@postgrespro.ru>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: David Steele <david@pgmasters.net>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Oleg Bartunov <obartunov@gmail.com>
Cc: Pavel Stehule <pavel.stehule@gmail.com>
Subject: Re: jsonpath
Date: Mon, 21 Jan 2019 16:56:46 +0300
Message-ID: <a37b47a8-d853-b9ba-9b61-2e87ecc783b2@postgrespro.ru> (raw)
In-Reply-To: <CAPpHfdtq0b_g9MmDPpAOFkxXSH93TrJCkxNc5EmJktUJ0tuZtA@mail.gmail.com>
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On 20.01.2019 2:45, Alexander Korotkov wrote:

> 3) How do we calculate the "id" property returned by keyvalue()
> function?  It's not documented.  Even presence of "id" columns isn't
> documented.  Standard stands that it's implementation-depended
> indetifier of object holding key-value pair.  The way of its
> calculation is also not clear from the code.  Why do we need constant
> of 10000000000?
>
>                  id = jb->type != jbvBinary ? 0 :
>                      (int64)((char *) jb->val.binary.data -
>                              (char *) cxt->baseObject.jbc);
>                  id += (int64) cxt->baseObject.id * INT64CONST(10000000000);

I decided to construct object id from the two parts: base object id and its
binary offset in its base object's jsonb:

object_id = 10000000000 * base_object_id + object_offset_in_base_object


10000000000 (10^10) -- is a first round decimal number greater than 2^32
(maximal offset in jsonb).  Decimal multiplier is used here to improve the
readability of identifiers.


Base object is usually a root object of the path: context item '$' or path
variable '$var', literals can't produce objects for now.  But if the path
contains generated objects (.keyvalue() itself, for example), then they become
base object for the subsequent .keyvalue().  See example:

'$.a.b.keyvalue().value.keyvalue()' :
  - base for the first .keyvalue() is '$'
  - base for the second .keyvalue() is '$.a.b.keyvalue()'


Id of '$' is 0.
Id of '$var' is its ordinal (positive) number in the list of variables.
Ids for generated objects are assigned using global counter
'JsonPathExecContext.generatedObjectId' starting from 'number_of_vars + 1'.


Corresponding comments will be added in the upcoming version of the patches.

-- 
Nikita Glukhov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

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  Cc: n.gluhov@postgrespro.ru, a.korotkov@postgrespro.ru, tomas.vondra@2ndquadrant.com, michael@paquier.xyz, s.kelvich@postgrespro.ru, thomas.munro@enterprisedb.com, david@pgmasters.net, tgl@sss.pgh.pa.us, andrew.dunstan@2ndquadrant.com, robertmhaas@gmail.com, obartunov@gmail.com, pavel.stehule@gmail.com
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