docs: add notes on contents of Postgres DB as Key-Value store to Postgres.md

[84adam]

Change Description

Adding notes on how Postgres DB is currently used as a Key-Value store, and which tables are included.

Addresses Issue #6619 - "Missing documentation for postgresql database structure"

Steps to Test

Key/Value pairs are not all TLV-encoded. Schema/structure is unclear (outside of K/V column names). However, some Key Names and Values are possible to decode into human-readable strings. For example: I used Python + psycopg2 + text processing to decode some of these, such as Node names (see comment on Gist): https://gist.github.com/84adam/d85f5f40e8821925585f263f68b655eb

SQL statements do not reveal any consistent encoding or structure:

select * from channeldb_kv limit 5;
                  key                   | value | parent_id | id | sequence 
----------------------------------------+-------+-----------+----+----------
 \x6f70656e2d6368616e2d6275636b6574     |       |           |  1 |         
 \x636c6f7365642d6368616e2d6275636b6574 |       |           |  2 |         
 \x636972637569742d6677642d6c6f67       |       |           |  3 |         
 \x6677642d7061636b61676573             |       |           |  4 |         
 \x696e766f69636573                     |       |           |  5 |         

select * from walletdb_kv limit 5;
        key         | value | parent_id | id | sequence 
--------------------+-------+-----------+----+----------
 \x77616464726d6772 |       |           |  1 |         
 \x7774786d6772     |       |           |  2 |         
 \x6d61696e         |       |         1 |  3 |         
 \x73796e63         |       |         1 |  4 |         
 \x73636f7065       |       |         1 |  5 | 

This documentation update attempts to make clear that users cannot expect consistency for now in issuing SQL queries to their Postgres databases, and points out where more documentation will be required once additional schema is introduced into these tables.

[ellemouton]

Thanks for the PR @84adam!

Pls see our contributor guidelines for how to structure your commit title.

So would suggest something like: docs: add <short description here>

Also please add an entry to the release notes

[84adam]

Thank you, @ellemouton. Updated.

[yyforyongyu]

Thanks for the PR! @ellemouton 's comment is not addressed thought,

So would suggest something like: docs: add

So the git commit messages need to be updated as suggested.

[84adam]

Thanks, @yyforyongyu. I believe I have corrected this now.

[84adam]

@yyforyongyu I believe the requested changes have been completed. (Squashed commits.) Please let me know if I'm missing anything else. Thanks!

[84adam]

Removed previous "references" and squashed commits.

- https://twitter.com/positiveblue2/status/1621170766808256512 - "Currently LND supports Postgres but it is used as K/V store. Introducing better schemas will unlock better performance and make easier to get insights from our nodes."
- https://twitter.com/lndreviewclub/status/1618696557402152961 - "Postgres support was added in lnd 0.14, but the next step is to convert the invoices to a native SQL schema."
- https://lightningcommunity.slack.com/archives/C03ECAP7HK6/p1675357998163889?thread_ts=1675357814.564459&cid=C03ECAP7HK6 - 'Values are a mix of TLV and non-TLV-encoded data. Key/Values are just stored as plain byte slices. With more schema introduced it'll gain a lot of performance over time. This methodology allows for incrementally upgrading the DB code over time. Invoice DB transactions do not touch data in other buckets (they are self contained).'
- https://gist.github.com/84adam/d85f5f40e8821925585f263f68b655eb - Python script to decode keys/values from Postgres LND database

[yyforyongyu]

LGTM✅ Thanks again for the PR!

[ellemouton]

Looks good! I think it could be worth adding an entry to sqlite.md too. Or at least linking to this explanation from there?

[84adam]

Anything I'm missing for this PR? Curious if you have any idea when it might be merged? @ellemouton @yyforyongyu

Thanks!

[yyforyongyu]

@84adam added to 0.16.1 so this will be merged before that release.