Re: Why is Hash index not transaction safe.

HI all, I have the file postmaster.pid - I would like to know
what the lines mean? I did Google, but didn't find much.

======================================
[pol@localhost inst]$ more ./data/postmaster.pid
7382
/home/pol/Downloads/software/postgres/inst/./data
1430769205
5432
/tmp
localhost
  5432001   2195471
[pol@localhost inst]$
=======================================

"inst" is the base install of the PostgreSQL instance.

7382 is the process id of the .../inst/bin/postgres -D ./data process

I'm curious as to what this line
/home/pol/Downloads/software/postgres/inst/./data
means
The PostgreSQL install is in /home/pol/Downloads/software/postgres/inst/, but
what does the bit of the path inst/./data - I'm unclear as to what the . (dot)
in the middle of that path means.

Is the 1430769205 a UNIX epoch time (seconds since 1970) since the
instance started?

5432 is the port (configurable)

/tmp - the PostgreSQL temp (sorting) directory?

localhost  is my machine name (never bothered to give it anything meaningful).

It's the last line
  5432001   2195471
that really puzzled me - I don't have a clue what they are about.

I'd be grateful for any input on this matter,


TIA and rgs,


Pól Ua Laoínecháin



2015-05-04 5:11 GMT+01:00 Tom Lane <tgl@sss.pgh.pa.us>:
> Wei Shan <weishan.ang@gmail.com> writes:
>> I read the following about Hash indexes in Heroku's blog (
>> https://devcenter.heroku.com/articles/postgresql-indexes)
>
>> *Hash Indexes are only useful for equality comparisons, but you pretty much
>> never want to use them since they are not transaction safe, need to be
>> manually rebuilt after crashes, and are not replicated to followers, so the
>> advantage over using a B-Tree is rather small.*
>
>> Could anyone explain about why is it not transaction safe as compared to
>> B-Tree index.
>
> They're not crash-safe because they don't have any WAL support, and
> WAL-based replication doesn't work for the same reason.  But I think
> the bit about not being transaction-safe is nonsense ...
>
>                         regards, tom lane
>
>
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