Markdown format output for psql, design notes

Hi all,

# What I'd like to do
I've been working on the idea of a markdown format for psql as I had said
in that thread :
https://www.postgresql.org/message-id/flat/CAB_COdiiwTmBcrmjXWCKiqkcPgf_bLodrUyb4GYE6pfKeoK2eg%40mail.gmail.com

An attempt was made a year ago (see here :
https://www.postgresql.org/message-id/flat/CAAYBy8bs%3D8vz6Ps_nLW24NJhqcxz4bsWBLawAiwWSPSLdWSmvA%40mail.gmail.com#eb7b6eb6daa60aac1f5fa001f934f89a),
but didn't end up with something commitable.

What's more, I quite disagree with `\pset linestyle markdown` option to
have a markdown output in psql, I prefer `\pset format markdown`.
# Some official doc about markdown So here are my thoughts (before writing
any code) :

1. "Official" markdown seems to be the daring fireball project (see
Aaron Schwartz's note here http://www.aaronsw.com/weblog/001189)
2. Official markdown doesn't support table formatting. Authors said we
could just use HTML inside markdown to do so (it's quite not readable for a
human, that's why I don't like this option) -> see here:
https://daringfireball.net/projects/markdown/syntax#html
3. Table markdown is introduced in "Markdown Extra" that was first
implemented in PHP (see here
https://michelf.ca/projects/php-markdown/extra/#table)
4. I want to make the patch as simple as possible, so I won't implement
cell alignment

# The result I want
From points 3 and 4, here is what I'd like to see :

| Header 1 | Header 2 | Header 3 |
|----------|----------|----------|
| content | content | content |
| content | content | content |
(2 rows)

***'|' at beginning and end of line are optional in markdown extra, but it
seems as a consensus to always add them. You may challenge this choice, I'm
open to discussion.***

From the fireball project (
https://daringfireball.net/projects/markdown/syntax#backslash) and markdown
extra (https://michelf.ca/projects/php-markdown/extra/#backslash), it seems
we need to backslash escape all of those characters:

~~~
\ backslash
` backtick
* asterisk
_ underscore
{} curly braces
[] square brackets
() parentheses
# hash mark
+ plus sign
- minus sign (hyphen)
. dot
! exclamation mark
: colon
| pipe
~~~

# psql syntax to get that
It feels to me that we should use the `\pset format` (or `-P` or `--pset=`
in batch mode) syntax to tell psql we want markdown. So any of that one
should provide a markdown output :

- `psql -P format=markdown`
- `psql --pset=format=markdown`
- `\pset format markdown` (in psql prompt command)

# Code to change
If I want to code that patch, here are the files I think I'll need to
change :

- Documentation
- doc/src/sgml/rel/psql-ref.sgml
- src/bin/psql/help.c
- Tests
- src/test/regress/expected/psql.out
- src/test/regress/sql/psql.sql
- Code
- src/bin/psql/command.c
- src/bin/psql/tab-complete.c

***You're welcome to add any other file that I missed in that list!***

# What I'd like you to do
First, thanks to have read that whole mail and sorry I didn't mean to make
it so long...
Then I'd like to know ***what you think about what I'm about to do***
before heading in a wrong direction.

Have a nice day,

Lætitia
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