Re: pg_walinspect - a new extension to get raw WAL data and WAL stats

Hi Bharath,

First look at the patch, bear with me if any of the following comments are
repeated.
1. With pg_get_wal_record(lsn), say a WAL record start, end lsn range
contains the specified LSN, wouldn't it be more meaningful to show the
corresponding WAL record.
For example, upon providing '0/17335E7' as input, and I see get the WAL
record ('0/1733618', '0/173409F') as output and not the one with start and
end lsn as ('0/17335E0', '0/1733617').

With pg_walfile_name(lsn), we can find the WAL segment file name that
contains the specified LSN.

2. I see the following output for pg_get_wal_record. Need to have a look at
the spaces I suppose.
rkn=# select * from pg_get_wal_record('0/4041728');
start_lsn | end_lsn | prev_lsn | record_length |

record

-----------+-----------+-----------+---------------+---------------------------------------------------------------------------------------------------------------------
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0/4041728 | 0/40421AF | 0/40416F0 | 2670 |
\x6e0a0000d2020000f016040400000000000a0000fef802b400007f7f00000000408fe738a25500000300000000000000010000007f06000000
4000003b0a00000000000012000000100101007f7f7f7f0885e738a25500003000c815380a03000885e738a255000000007f7f7f7f7f7f0000000078674301296000003000f8150020042000000000709e1203909
dba01c89c8601609cd00030986008f8956804d202000000000000000000000000120006001f00030820ffff5f04000000000001400000010000000000000004000000000080bf0200030000000000000000006100
0000610000000000000000000000000000000000000000000000000000000000000000000000330100000000000000bc02000001000000010000000000803f00000000000000b0060000010000000000000017000

3. Should these functions be running in standby mode too? We do not allow
WAL control functions to be executed during recovery right?

4. If the wal segment corresponding to the start lsn is removed, but there
are WAL records which could be read in the specified input lsn range, would
it be better to output the existing WAL records displaying a message that
it is a partial list of WAL records and the WAL files corresponding to the
rest are already removed, rather than erroring out saying "requested WAL
segment has already been removed"?

5. Following are very minor comments in the code

- Correct the function description by removing "return the LSN up to
which the server has WAL" for IsFutureLSN
- In GetXLogRecordInfo, good to have pfree in place for rec_desc,
rec_blk_ref, data
- In GetXLogRecordInfo, can avoid calling XLogRecGetInfo(record)
multiple times by capturing in a variable
- In GetWALDetailsGuts, setting end_lsn could be done in single if else
and similarly we can club the if statements verifying if the start lsn is a
future lsn.

Thanks,
RKN