Обсуждение: Building multiple indexes concurrently
Lets say I have a large table bigTable to which I would like to add two btree indexes. Is there a more efficient way to create indexes than: CREATE INDEX idx_foo on bigTable (foo); CREATE INDEX idx_baz on bigTable (baz); Or CREATE INDEX CONCURRENTLY idx_foo on bigTable (foo); CREATE INDEX CONCURRENTLY idx_baz on bigTable (baz); Are there any particular performance optimizations that would be in play in such a scenario? At a minimum I assume that if both of the commands were started at about the same time they would each scan the table in the same direction and whichever creation was slower would benefit from most of the table data it needed being prepopulated in shared buffers. Is this the case? -- Rob Wultsch wultsch@gmail.com
On Mar 16, 2010, at 6:04 PM, Rob Wultsch wrote: > Lets say I have a large table bigTable to which I would like to add > two btree indexes. Is there a more efficient way to create indexes > than: > CREATE INDEX idx_foo on bigTable (foo); > CREATE INDEX idx_baz on bigTable (baz); > Or > CREATE INDEX CONCURRENTLY idx_foo on bigTable (foo); > CREATE INDEX CONCURRENTLY idx_baz on bigTable (baz); > > Are there any particular performance optimizations that would be in > play in such a scenario? > > At a minimum I assume that if both of the commands were started at > about the same time they would each scan the table in the same > direction and whichever creation was slower would benefit from most of > the table data it needed being prepopulated in shared buffers. Is this > the case? That sounds reasonable to me. You might also look at upping your maintenance_work_mem for your session, as well.
Rob Wultsch wrote: > Are there any particular performance optimizations that would be in > play in such a scenario? > You'd want to increase maintenance_work_mem significantly, just for the sessions that are running these. Something like this: |SET maintenance_work_mem = '1GB';| I don't know if that's a huge or tiny number relative to total RAM in your server, you get the idea though. Also, you should have a larger than default value for checkpoint_segments in advance of this. That you can't set per session, but you can adjust the value in the postgresql.conf and request a configuration reload--don't actually need to disrupt server operation by restarting to do it. This will work for that: pg_ctl reload > At a minimum I assume that if both of the commands were started at > about the same time they would each scan the table in the same > direction and whichever creation was slower would benefit from most of > the table data it needed being prepopulated in shared buffers. Is this > the case? > This might be optimistic; whether it will be the case depends a lot on how large your shared_buffers and OS buffer cache are relative to the table involved. To pick an extreme example to demonstrate what I mean, if shared_buffers is the common default of <32MB, your table is 1TB, and you have a giant disk array that reads fast, it's not very likely that the second scan is going to find anything of interest left behind by the first one. You could try and make some rough estimates of how long it will take to fill your RAM with table data at the expected I/O rate and guess how likely overlap is. There's a trade-off here, which is that in return for making it possible the data you need to rebuild the index is more likely to be in RAM when you need it by building two at once, the resulting indexes are likely to end up interleaved on disk as they are written out. If you're doing a lot of index scans, the increased seek penalties for that may ultimately make you regret having combined the two. Really impossible to predict which approach is going to be better long term without gathering so much data that you might as well try and benchmark it on a test system instead if you can instead. I am not a big fan of presuming one can predict performance instead of measuring it for complicated cases. -- Greg Smith 2ndQuadrant US Baltimore, MD PostgreSQL Training, Services and Support greg@2ndQuadrant.com www.2ndQuadrant.us
Greg Smith <greg@2ndquadrant.com> writes:
> Rob Wultsch wrote:
>> At a minimum I assume that if both of the commands were started at
>> about the same time they would each scan the table in the same
>> direction and whichever creation was slower would benefit from most of
>> the table data it needed being prepopulated in shared buffers. Is this
>> the case?
> This might be optimistic;
No, it's not optimistic in the least, at least not since we implemented
synchronized seqscans (in 8.3 or thereabouts).
regards, tom lane
On Wed, Mar 17, 2010 at 7:30 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > Greg Smith <greg@2ndquadrant.com> writes: >> Rob Wultsch wrote: >>> At a minimum I assume that if both of the commands were started at >>> about the same time they would each scan the table in the same >>> direction and whichever creation was slower would benefit from most of >>> the table data it needed being prepopulated in shared buffers. Is this >>> the case? > >> This might be optimistic; > > No, it's not optimistic in the least, at least not since we implemented > synchronized seqscans (in 8.3 or thereabouts). > > regards, tom lane > Where can I find details about this in the documentation? -- Rob Wultsch wultsch@gmail.com
Rob Wultsch wrote: > On Wed, Mar 17, 2010 at 7:30 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > >> No, it's not optimistic in the least, at least not since we implemented >> synchronized seqscans (in 8.3 or thereabouts). >> > Where can I find details about this in the documentation? > It's a behind the scenes optimization so it's not really documented on the user side very well as far as I know; easy to forget it's even there as I did this morning. http://j-davis.com/postgresql/syncscan/syncscan.pdf is a presentation covering it, and http://j-davis.com/postgresql/83v82_scans.html is also helpful. While my pessimism on this part may have been overwrought, note the message interleaved on the list today with this discussion from Bob Lunney discussing the other issue I brought up: "When using 8-way parallel restore against a six-disk RAID 10 group I found that table and index scan performance dropped by about 10x. I/O performance was restored by either clustering the tables one at a time, or by dropping and restoring them one at a time. The only reason I can come up with for this behavior is file fragmentation and increased seek times." Now, Bob's situation may very well involve a heavy dose of table fragmentation from multiple active loading processes rather than index fragmentation, but this class of problem is common when trying to do too many things at the same time. I'd hate to see you chase a short-term optimization (reduce total index built time) at the expense of long-term overhead (resulting indexes are not as efficient to scan). -- Greg Smith 2ndQuadrant US Baltimore, MD PostgreSQL Training, Services and Support greg@2ndQuadrant.com www.2ndQuadrant.us
On Wednesday 17 March 2010 19:44:56 Greg Smith wrote: > Rob Wultsch wrote: > > On Wed, Mar 17, 2010 at 7:30 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > >> No, it's not optimistic in the least, at least not since we implemented > >> synchronized seqscans (in 8.3 or thereabouts). > > > > Where can I find details about this in the documentation? > > It's a behind the scenes optimization so it's not really documented on > the user side very well as far as I know; easy to forget it's even there > as I did this morning. > http://j-davis.com/postgresql/syncscan/syncscan.pdf is a presentation > covering it, and http://j-davis.com/postgresql/83v82_scans.html is also > helpful. > > While my pessimism on this part may have been overwrought, note the > message interleaved on the list today with this discussion from Bob > Lunney discussing the other issue I brought up: "When using 8-way > parallel restore against a six-disk RAID 10 group I found that table and > index scan performance dropped by about 10x. I/O performance was > restored by either clustering the tables one at a time, or by dropping > and restoring them one at a time. The only reason I can come up with > for this behavior is file fragmentation and increased seek times." Now, > Bob's situation may very well involve a heavy dose of table > fragmentation from multiple active loading processes rather than index > fragmentation, but this class of problem is common when trying to do too > many things at the same time. I'd hate to see you chase a short-term > optimization (reduce total index built time) at the expense of long-term > overhead (resulting indexes are not as efficient to scan). I find it way much easier to believe such issues exist on a tables in constrast to indexes. The likelihood to get sequential accesses on an index is small enough on a big table to make it unlikely to matter much. Whats your theory to make it matter much? Andres
Andres Freund escribió: > I find it way much easier to believe such issues exist on a tables in > constrast to indexes. The likelihood to get sequential accesses on an index is > small enough on a big table to make it unlikely to matter much. Vacuum walks indexes sequentially, for one. -- Alvaro Herrera http://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support
Alvaro Herrera wrote: > Andres Freund escribió: > > >> I find it way much easier to believe such issues exist on a tables in >> constrast to indexes. The likelihood to get sequential accesses on an index is >> small enough on a big table to make it unlikely to matter much. >> > > Vacuum walks indexes sequentially, for one. > That and index-based range scans were the main two use-cases I was concerned would be degraded by interleaving index builds, compared with doing them in succession. I work often with time-oriented apps that have heavy "give me every record between <a> and <b>" components to them, and good sequential index performance can be an important requirement for that kind of application. -- Greg Smith 2ndQuadrant US Baltimore, MD PostgreSQL Training, Services and Support greg@2ndQuadrant.com www.2ndQuadrant.us
On Wed, 2010-03-17 at 16:49 -0400, Greg Smith wrote: > Alvaro Herrera wrote: > > Andres Freund escribió: > > > > > >> I find it way much easier to believe such issues exist on a tables in > >> constrast to indexes. The likelihood to get sequential accesses on an index is > >> small enough on a big table to make it unlikely to matter much. > >> > > > > Vacuum walks indexes sequentially, for one. > > > > That and index-based range scans were the main two use-cases I was > concerned would be degraded by interleaving index builds, compared with > doing them in succession. I guess that tweaking file systems to allocate in bigger chunks help here ? I know that xfs can be tuned in that regard, but how about other common file systems like ext3 ? - Hannu Krosing http://www.2ndQuadrant.com PostgreSQL Scalability and Availability Services, Consulting and Training
On Wednesday 17 March 2010 22:18:47 Hannu Krosing wrote: > On Wed, 2010-03-17 at 16:49 -0400, Greg Smith wrote: > > Alvaro Herrera wrote: > > > Andres Freund escribió: > > >> I find it way much easier to believe such issues exist on a tables in > > >> constrast to indexes. The likelihood to get sequential accesses on an > > >> index is small enough on a big table to make it unlikely to matter > > >> much. > > > > > > Vacuum walks indexes sequentially, for one. > > > > That and index-based range scans were the main two use-cases I was > > concerned would be degraded by interleaving index builds, compared with > > doing them in succession. > > I guess that tweaking file systems to allocate in bigger chunks help > here ? I know that xfs can be tuned in that regard, but how about other > common file systems like ext3 ? ext4 should do that now by allocating the space for the files only after some time or uppon things like fsync (xfs does the same). ext3 has, as far as I know, neither the ability to change allocation size nor can do delayed allocation. Andres
It seems to me that a separate partition / tablespace would be a much simpler approach. On Mar 17, 2010, at 5:18 PM, Hannu Krosing wrote: > On Wed, 2010-03-17 at 16:49 -0400, Greg Smith wrote: >> Alvaro Herrera wrote: >>> Andres Freund escribió: >>> >>> >>>> I find it way much easier to believe such issues exist on a tables in >>>> constrast to indexes. The likelihood to get sequential accesses on an index is >>>> small enough on a big table to make it unlikely to matter much. >>>> >>> >>> Vacuum walks indexes sequentially, for one. >>> >> >> That and index-based range scans were the main two use-cases I was >> concerned would be degraded by interleaving index builds, compared with >> doing them in succession. > > I guess that tweaking file systems to allocate in bigger chunks help > here ? I know that xfs can be tuned in that regard, but how about other > common file systems like ext3 ? > > - > Hannu Krosing http://www.2ndQuadrant.com > PostgreSQL Scalability and Availability > Services, Consulting and Training > > > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance
On Thu, 2010-03-18 at 16:12 -0400, Justin Pitts wrote: > It seems to me that a separate partition / tablespace would be a much simpler approach. Do you mean a separate partition/ tablespace for _each_ index built concurrently ? > On Mar 17, 2010, at 5:18 PM, Hannu Krosing wrote: > > > On Wed, 2010-03-17 at 16:49 -0400, Greg Smith wrote: > >> Alvaro Herrera wrote: > >>> Andres Freund escribió: > >>> > >>> > >>>> I find it way much easier to believe such issues exist on a tables in > >>>> constrast to indexes. The likelihood to get sequential accesses on an index is > >>>> small enough on a big table to make it unlikely to matter much. > >>>> > >>> > >>> Vacuum walks indexes sequentially, for one. > >>> > >> > >> That and index-based range scans were the main two use-cases I was > >> concerned would be degraded by interleaving index builds, compared with > >> doing them in succession. > > > > I guess that tweaking file systems to allocate in bigger chunks help > > here ? I know that xfs can be tuned in that regard, but how about other > > common file systems like ext3 ? > > > > - > > Hannu Krosing http://www.2ndQuadrant.com > > PostgreSQL Scalability and Availability > > Services, Consulting and Training > > > > > > > > -- > > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > > To make changes to your subscription: > > http://www.postgresql.org/mailpref/pgsql-performance > -- Hannu Krosing http://www.2ndQuadrant.com PostgreSQL Scalability and Availability Services, Consulting and Training
Yes. On Mar 18, 2010, at 5:20 PM, Hannu Krosing wrote: > On Thu, 2010-03-18 at 16:12 -0400, Justin Pitts wrote: >> It seems to me that a separate partition / tablespace would be a much simpler approach. > > Do you mean a separate partition/ tablespace for _each_ index built > concurrently ?