Re: problem with precendence order in JSONB merge operator

On Tue, Mar 22, 2016 at 1:52 PM, Peter Krauss <ppkrauss@gmail.com> wrote:

Seems that parser not using precedence ideal order, and that casting obligation losts performance.

  

The first problem is self-evident in this example:

SELECT '{"x":1}'::jsonb || (('{"A":{"y":2}}'::jsonb)->'A')  -- it is ok, expected result with (x,y)

SELECT '{"x":1}'::jsonb || '{"A":{"y":2}}'::jsonb)->'A'    -- non-expected result (y).

Higher precedence most be for -> operator, that is like an object-oriented path operator, always higher than algebric ones.

​There is presently no formal concept of "path operator" in PostgreSQL.  "->" is a user-defined operator, as is "||"​ and thus have equal precedence and left associativity.

http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html

Regardless, "||" is not an "algebric" [sic] operator...I'm curious what source you are using to back your claim of operator precedence between different so-called "operator types".

Its highly undesirable to make changes to operator precedence.

Operators are simply symbols to the parser - there is no context involved that would allow making their precedence dynamic.  So all PostgreSQL sees is "||", not a "JSONB merge operator".

Other problem is using this operation as SQL function

  CREATE FUNCTION term_lib.junpack(jsonb,text) RETURNS JSONB AS $f$       SELECT ($1-$2)::JSONB || ($1->>$2)::JSONB;  $f$ LANGUAGE SQL IMMUTABLE;

without casting produce error. Perhaps will be "more friendly" without cast obligation, 

and it is a performance problem, the abusive use of castings losts performance.

I cannot make this work...

version

PostgreSQL 9.5.1 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2, 64-bit

SELECT ('{"a":1,"b":2}'::jsonb - 'b'::text)::jsonb || ('{"a":1,"b":2}'::jsonb #> 'b'::text)::jsonb

> ​SQL Error: ERROR: invalid concatenation of jsonb objects

​

This seems like user error but without a self-contained test case exercising the query (the use of a function in this context should be immaterial) I'm finding it hard to explain why.  My simple case returns a non-object with rightly cannot be appended to an object.

In isolatoin you can avoid casting the RHS of the || operator by using the "#>(jsonb,text[])" operator

SELECT pg_typeof('{"a":1,"b":{"c":2}}'::jsonb #> array['b']::text[]) --jsonb

JSON, IME, still needs some fleshing out.  Efficient usage might require additional features but for now one needs to get very familiar with all the various operator variants that allow the user to choose whether to return json or text and to pick the correct one for their needs.

​David J.

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