Re: transactions start time

[Aleksei Arefjev <aleksei.arefjev@xxxxxxxxxxxxxxxx>]

On 24 July 2012 20:21, Richard Huxton <dev@xxxxxxxxxxxx> wrote:

On 24/07/12 12:14, Aleksei Arefjev wrote:

Hi,

In statistical reports gathered by PgBadger on our PostgreSQL databases
almost always we have in "Queries that took up the most time" report
table information about transactions start time ('BEGIN;' command).
Something like that in example below:

2    3h34m52.26s    48,556,167    0.00s    BEGIN;

                         0.82s | BEGIN;
                         0.82s | BEGIN;
                         0.82s | BEGIN;
                         0.81s | BEGIN;
                         0.81s | BEGIN;
                         0.81s | BEGIN;
                         0.80s | BEGIN;
                         0.80s | BEGIN;
                         0.79s | BEGIN;
                         0.79s | BEGIN;

I'm not sure if I'm reading this right, but are there more than 48 million BEGINs that took 0s each (presumably rounded down) and then a handful taking about 0.8s?

If so, then it's likely nothing to do with the BEGIN and just that the machine was busy doing other things when you started a transaction.

Databases placed on different hardware, OS - Debian GNU/Linux,
PostgreSQL 9.1

So, questions are:
1. Is this a normal situation with transactions start time ( BEGIN method) ?

See above

2. How can we reduce transactions start time if it's possible in principle?

Below 0.00? Probably not

3. What happens in PostgreSQL on transaction starting time? Can someone
describe this process in detail? (of course, I saw in PostgreSQL source
code, for example, definition such kind functions, like StartTransaction
function, but it's not so easy to understand for third-party researcher,
that all of these operations mean in real for performance)

Well there are two important things to understand:
1. All* commands run in a transaction
2. I think most of the work in getting a new snapshot etc gets pushed back until it's needed.

If so, maybe using of 'SET TRANSACTION SNAPSHOT' command with the pre-existing transaction exported snapshot by the pg_export_snapshot function could be usefull for reducing transactions start time -
http://www.postgresql.org/docs/9.2/static/sql-set-transaction.html
 

So - the overall impact of issuing BEGIN should be close to zero.

--
  Richard Huxton
  Archonet Ltd