>>ISTM that "source" is worth knowing.
>
> Hm, possibly.  Any other opinions?

This version has the seven fields I proposed, including "source". Here's
an example that shows why I think it's valuable:

regression=# \x
Expanded display is on.
regression=# select * from pg_settings where name = 'enable_seqscan';
-[ RECORD 1 ]-----------
name    | enable_seqscan
setting | on
context | user
vartype | bool
source  | default
min_val |
max_val |

regression=# update pg_settings set setting = 'off' where name =
'enable_seqscan';
-[ RECORD 1 ]---
set_config | off

regression=# select * from pg_settings where name = 'enable_seqscan';
-[ RECORD 1 ]-----------
name    | enable_seqscan
setting | off
context | user
vartype | bool
source  | session
min_val |
max_val |

regression=# alter user postgres set enable_seqscan to 'off';
ALTER USER

(log out and then back in again)

regression=# \x
Expanded display is on.
regression=# select * from pg_settings where name = 'enable_seqscan';
-[ RECORD 1 ]-----------
name    | enable_seqscan
setting | off
context | user
vartype | bool
source  | user
min_val |
max_val |

In the first case, enable_seqscan is set to its default value. After
setting it to off, it is obvious that the value has been changed for the
session only. In the third case, you can see that the value has been set
specifically for the user.

Joe Conway