(WIP) Add docs for json_table

[mosabua]

Description

Not even really started .. just getting my thoughts together. Currently I plan to add this to the json functions page .. and maybe also link from the table functions page .. will see how it goes.

Will rely on specs, tests, and help from @kasiafi and @martint .. but not yet.

Additional context and related issues

• First released in 435 with Add support for json_table #18017
• Fix for being able to test it at all is in 436 - Fix serialization failure when invoking json_table #20122

Release notes

(x) This is not user-visible or is docs only, and no release notes are required.

[findinpath]

One example which i put together:

trino> SELECT
    ->   *
    -> FROM
    ->   json_table (
    ->     '[{"id":1,"name":"Africa","wikiDataId":"Q15"},{"id":2,"name":"Americas","wikiDataId":"Q828"},{"id":3,"name":"Asia","wikiDataId":"Q48"},{"id":4,"name":"Europe","wikiDataId":"Q51"}]',
    ->     'lax $' COLUMNS (
    ->         NESTED PATH 'lax $[*]' AS nested_path COLUMNS (
    ->             id integer path 'lax $[*].id',
    ->             name varchar path 'lax $[*].name',
    ->             wiki_data_id varchar path 'lax $[*]."wikiDataId"')));
 ID |   NAME   | WIKI_DATA_ID 
----+----------+--------------
  1 | Africa   | Q15          
  2 | Americas | Q828         
  3 | Asia     | Q48          
  4 | Europe   | Q51          
(4 rows)

[martint]

You don't need the [*] in $[*].id, $[*].name and '$[*].wikiDataId, since the elements of the top-level array are not arrays. It happens to work because you're using lax`, which does some conversions automatically and is more lenient. This would be the proper form:

SELECT
  *
FROM
  json_table (
	'[{"id":1,"name":"Africa","wikiDataId":"Q15"},{"id":2,"name":"Americas","wikiDataId":"Q828"},{"id":3,"name":"Asia","wikiDataId":"Q48"},{"id":4,"name":"Europe","wikiDataId":"Q51"}]',
	'strict $' COLUMNS (
		NESTED PATH 'strict $[*]' AS nested_path COLUMNS (
			id integer path 'strict $.id',
			name varchar path 'strict $.name',
			wiki_data_id varchar path 'strict $."wikiDataId"')));

[mosabua]

Thanks @findinpath and @martint .. I barely started but I will look at the various test cases and such to create some examples as well as details the different sections of the table function.

[mosabua]

Replaced by PR from @michaeleby1