patch: xmltable - proof concept

Hi

I am sending a initial implementation of xmltable function:

The code is not clean now, but it does almost of expected work. The usage
is simple. It is fast - 16K entries in 400ms.

I invite any help with documentation and testing.

The full ANSI/SQL, or Oracle compatible implementation is not possible due
limits of libxml2, but for typical usage it should to work well. It doesn't
need any new reserved keyword, so there should not be hard barriers for
accepting (when this work will be complete).

Example:

postgres=# SELECT * FROM xmldata;
┌──────────────────────────────────────────────────────────────────┐
│ data │
╞══════════════════════════════════════════════════════════════════╡
│ <ROWS> ↵│
│ <ROW id="1"> ↵│
│ <COUNTRY_ID>AU</COUNTRY_ID> ↵│
│ <COUNTRY_NAME>Australia</COUNTRY_NAME> ↵│
│ <REGION_ID>3</REGION_ID> ↵│
│ </ROW> ↵│
│ <ROW id="2"> ↵│
│ <COUNTRY_ID>CN</COUNTRY_ID> ↵│
│ <COUNTRY_NAME>China</COUNTRY_NAME> ↵│
│ <REGION_ID>3</REGION_ID> ↵│
│ </ROW> ↵│
│ <ROW id="3"> ↵│
│ <COUNTRY_ID>HK</COUNTRY_ID> ↵│
│ <COUNTRY_NAME>HongKong</COUNTRY_NAME> ↵│
│ <REGION_ID>3</REGION_ID> ↵│
│ </ROW> ↵│
│ <ROW id="4"> ↵│
│ <COUNTRY_ID>IN</COUNTRY_ID> ↵│
│ <COUNTRY_NAME>India</COUNTRY_NAME> ↵│
│ <REGION_ID>3</REGION_ID> ↵│
│ </ROW> ↵│
│ <ROW id="5"> ↵│
│ <COUNTRY_ID>JP</COUNTRY_ID> ↵│
│ <COUNTRY_NAME>Japan</COUNTRY_NAME> ↵│
│ <REGION_ID>3</REGION_ID><PREMIER_NAME>Sinzo Abe</PREMIER_NAME>↵│
│ </ROW> ↵│
│ <ROW id="6"> ↵│
│ <COUNTRY_ID>SG</COUNTRY_ID> ↵│
│ <COUNTRY_NAME>Singapore</COUNTRY_NAME> ↵│
│ <REGION_ID>3</REGION_ID><SIZE unit="km">791</SIZE> ↵│
│ </ROW> ↵│
│ </ROWS> │
└──────────────────────────────────────────────────────────────────┘
(1 row)
postgres=# SELECT xmltable.*
postgres-# FROM (SELECT data FROM xmldata) x,
postgres-# LATERAL xmltable('/ROWS/ROW'
postgres(# PASSING data
postgres(# COLUMNS id int PATH '@id',
postgres(# country_name text PATH
'COUNTRY_NAME',
postgres(# country_id text PATH
'COUNTRY_ID',
postgres(# region_id int PATH 'REGION_ID',
postgres(# size float PATH 'SIZE',
postgres(# unit text PATH 'SIZE/@unit',
postgres(# premier_name text PATH
'PREMIER_NAME' DEFAULT 'not specified');
┌────┬──────────────┬────────────┬───────────┬──────┬──────┬───────────────┐
│ id │ country_name │ country_id │ region_id │ size │ unit │ premier_name │
╞════╪══════════════╪════════════╪═══════════╪══════╪══════╪═══════════════╡
│ 1 │ Australia │ AU │ 3 │ ¤ │ ¤ │ not specified │
│ 2 │ China │ CN │ 3 │ ¤ │ ¤ │ not specified │
│ 3 │ HongKong │ HK │ 3 │ ¤ │ ¤ │ not specified │
│ 4 │ India │ IN │ 3 │ ¤ │ ¤ │ not specified │
│ 5 │ Japan │ JP │ 3 │ ¤ │ ¤ │ Sinzo Abe │
│ 6 │ Singapore │ SG │ 3 │ 791 │ km │ not specified │
└────┴──────────────┴────────────┴───────────┴──────┴──────┴───────────────┘
(6 rows)

Regards

Pavel