Re: [POC] FETCH limited by bytes. - Mailing list pgsql-hackers

Hmm, somehow I removed some recipients, especially the
list. Sorry for the duplicate.

-----
Sorry, I've been back. Thank you for the comment.

> Do you have any insight into where I would pass the custom row fetches from
> the table struct to the scan struct?

Yeah it's one simple way to tune it, if the user knows the
appropreate value.

> Last year I was working on a patch to postgres_fdw where the fetch_size
> could be set at the table level and the server level.
> 
> I was able to get the settings parsed and they would show up in
> pg_foreign_table
> and pg_foreign_servers. Unfortunately, I'm not very familiar with how
> foreign data wrappers work, so I wasn't able to figure out how to get these
> custom values passed from the PgFdwRelationInfo struct into the
> query's PgFdwScanState
> struct.

Directly answering, the states needed to be shared among several
stages are holded within fdw_private. Your new variable
fpinfo->fetch_size can be read in postgresGetForeignPlan.  It
newly creates another fdw_private.  You can pass your values to
ForeignPlan making it hold the value there. Finally, you will get
it in postgresBeginForeginScan and can set it into
PgFdwScanState.

> I bring this up only because it might be a simpler solution, in that the
> table designer could set the fetch size very high for narrow tables, and
> lower or default for wider tables. It's also a very clean syntax, just
> another option on the table and/or server creation.
> 
> My incomplete patch is attached.

However, the fetch_size is not needed by planner (so far), so we
can simply read the options in postgresBeginForeignScan() and set
into PgFdwScanState. This runs once per exection.

Finally, the attached patch will work as you intended.

What do you think about this?

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center


> On Tue, Jan 27, 2015 at 4:24 AM, Kyotaro HORIGUCHI <
> horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> 
> > Thank you for the comment.
> >
> > The automatic way to determin the fetch_size looks become too
> > much for the purpose. An example of non-automatic way is a new
> > foreign table option like 'fetch_size' but this exposes the
> > inside too much... Which do you think is preferable?
> >
> > Thu, 22 Jan 2015 11:17:52 -0500, Tom Lane <tgl@sss.pgh.pa.us> wrote in <
> > 24503.1421943472@sss.pgh.pa.us>
> > > Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> writes:
> > > > Hello, as the discuttion on async fetching on postgres_fdw, FETCH
> > > > with data-size limitation would be useful to get memory usage
> > > > stability of postgres_fdw.
> > >
> > > > Is such a feature and syntax could be allowed to be added?
> > >
> > > This seems like a lot of work, and frankly an incredibly ugly API,
> > > for a benefit that is entirely hypothetical.  Have you got numbers
> > > showing any actual performance win for postgres_fdw?
> >
> > The API is a rush work to make the path for the new parameter
> > (but, yes, I did too much for the purpose that use from
> > postgres_fdw..)  and it can be any saner syntax but it's not the
> > time to do so yet.
> >
> > The data-size limitation, any size to limit, would give
> > significant gain especially for small sized rows.
> >
> > This patch began from the fact that it runs about twice faster
> > when fetch size = 10000 than 100.
> >
> >
> > http://www.postgresql.org/message-id/20150116.171849.109146500.horiguchi.kyotaro@lab.ntt.co.jp
> >
> > I took exec times to get 1M rows from localhost via postgres_fdw
> > and it showed the following numbers.
> >
> > =# SELECT a from ft1;
> > fetch_size,   avg row size(*1),   time,   alloced_mem/fetch(Mbytes)(*1)
> > (local)                            0.75s
> > 100            60                  6.2s       6000 (0.006)
> > 10000          60                  2.7s     600000 (0.6  )
> > 33333          60                  2.2s    1999980 (2.0  )
> > 66666          60                  2.4s    3999960 (4.0  )
> >
> > =# SELECT a, b, c from ft1;
> > fetch_size,   avg row size(*1),   time,   alloced_mem/fetch(Mbytes)(*1)
> > (local)                            0.8s
> > 100           204                 12  s      20400 (0.02 )
> > 1000          204                 10  s     204000 (0.2  )
> > 10000         204                  5.8s    2040000 (2    )
> > 20000         204                  5.9s    4080000 (4    )
> >
> > =# SELECT a, b, d from ft1;
> > fetch_size,   avg row size(*1),   time,   alloced_mem/fetch(Mbytes)(*1)
> > (local)                            0.8s
> > 100          1356                 17  s     135600 (0.136)
> > 1000         1356                 15  s    1356000 (1.356)
> > 1475         1356                 13  s    2000100 (2.0  )
> > 2950         1356                 13  s    4000200 (4.0  )
> >
> > The definitions of the environment are the following.
> >
> > CREATE SERVER sv1 FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host
> > 'localhost', dbname 'postgres');
> > CREATE USER MAPPING FOR PUBLIC SERVER sv1;
> > CREATE TABLE lt1 (a int, b timestamp, c text, d text);
> > CREATE FOREIGN TABLE ft1 (a int, b timestamp, c text, d text) SERVER sv1
> > OPTIONS (table_name 'lt1');
> > INSERT INTO lt1 (SELECT a, now(), repeat('x', 128), repeat('x', 1280) FROM
> > generate_series(0, 999999) a);
> >
> > The "avg row size" is alloced_mem/fetch_size and the alloced_mem
> > is the sum of HeapTuple[fetch_size] and (HEAPTUPLESIZE +
> > tup->t_len) for all stored tuples in the receiver side,
> > fetch_more_data() in postgres_fdw.
> >
> > They are about 50% gain for the smaller tuple size and 25% for
> > the larger. They looks to be optimal at where alloced_mem is
> > around 2MB by the reason unknown to me. Anyway the difference
> > seems to be significant.
> >
> > > Even if we wanted to do something like this, I strongly object to
> > > measuring size by heap_compute_data_size.  That's not a number that users
> > > would normally have any direct knowledge of; nor does it have anything
> > > at all to do with the claimed use-case, where what you'd really need to
> > > measure is bytes transmitted down the wire.  (The difference is not
> > small:
> > > for instance, toasted values would likely still be toasted at the point
> > > where you're measuring.)
> >
> > Sure. Finally, the attached patch #1 which does the following
> > things.
> >
> >  - Sender limits the number of tuples using the sum of the net
> >    length of the column values to be sent, not including protocol
> >    overhead. It is calculated in the added function
> >    slot_compute_attr_size(), using raw length for compressed
> >    values.
> >
> >  - postgres_fdw calculates fetch limit bytes by the following
> >    formula,
> >
> >    MAX_FETCH_MEM - MAX_FETCH_SIZE * (estimated overhead per tuple);
> >
> > The result of the patch is as follows. MAX_FETCH_MEM = 2MiB and
> > MAX_FETCH_SIZE = 30000.
> >
> > fetch_size,   avg row size(*1),   time,   max alloced_mem/fetch(Mbytes)
> > (auto)         60                  2.4s   1080000 ( 1.08)
> > (auto)        204                  7.3s    536400 ( 0.54)
> > (auto)       1356                 15  s    430236 ( 0.43)
> >
> > This is meaningfully fast but the patch looks too big and the
> > meaning of the new parameter is hard to understand..:(
> >
> >
> > On the other hand the cause of the displacements of alloced_mem
> > shown above is per-tuple overhead, the sum of which is unknown
> > before execution.  The second patch makes FETCH accept the tuple
> > overhead bytes. The result seems pretty good, but I think this
> > might be too spcialized to this usage.
> >
> > MAX_FETCH_SIZE = 30000 and MAX_FETCH_MEM = 2MiB,
> > max_fetch_size,   avg row size(*1),   time,   max
> > alloced_mem/fetch(MiBytes)
> > 30000             60                  2.3s   1080000 ( 1.0)
> >  9932            204                  5.7s   1787760 ( 1.7)
> >  1376           1356                 13  s   1847484 ( 1.8)
> >
> > MAX_FETCH_SIZE = 25000 and MAX_FETCH_MEM = 1MiB,
> > max_fetch_size,   avg row size(*1),   time,   max
> > alloced_mem/fetch(MiBytes)
> > 25000             60                  2.4s    900000 ( 0.86)
> >  4358            204                  6.6s    816840 ( 0.78)
> >   634           1356                 16  s    844488 ( 0.81)
> >
> > MAX_FETCH_SIZE = 10000 and MAX_FETCH_MEM = 0.5MiB,
> > max_fetch_size,   avg row size(*1),   time,   max
> > alloced_mem/fetch(MiBytes)
> > 10000             60                  2.8s    360000 ( 0.35)
> >  2376            204                  7.8s    427680 ( 0.41)
> >   332           1356                 17  s    442224 ( 0.42)
> >
> > regards,
> >
> > --
> > Kyotaro Horiguchi
> > NTT Open Source Software Center
> >
> >
> > --
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