[PERFORM] Index with all necessary columns - Postgres vs MSSQL
Hi,
I have a table in Postgres like:
CREATE TABLE test
(
id integer,
dtstamp timestamp without time zone,
rating real
)
CREATE INDEX test_all
ON test
USING btree
(id , dtstamp , rating);
My db has around 200M rows and I have reduced my test select statement down
to:
SELECT count(1) FROM test
WHERE id in (58,83,88,98,124,141,170,195,202,252,265,293,305,331,348)
AND dtstamp between cast('2011-10-19 08:00:00' as timestamp) and
cast('2011-10-19 16:00:00' as timestamp)
In Postgres this takes about 23 sec.
In MSSQL this takes about 1 sec.
MSSQL only accesses the index and does not access the table it self (uses
only index scan)
Postgres has the following plan:
"Aggregate (cost=130926.24..130926.25 rows=1 width=0)"
" -> Bitmap Heap Scan on test (cost=1298.97..130832.92 rows=37330
width=0)"
"Recheck Cond: ((id = ANY
('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
AND (dtstamp >= '2011-10-19 08:00:00'::timestamp without time zone) AND
(dtstamp <= '2011-10-19 16:00:00'::timestamp without time zone))"
"-> Bitmap Index Scan on test_all (cost=0.00..1289.64 rows=37330
width=0)"
" Index Cond: ((id = ANY
('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
AND (dtstamp >= '2011-10-19 08:00:00'::timestamp without time zone) AND
(dtstamp <= '2011-10-19 16:00:00'::timestamp without time zone))"
The results are disappointing since I want to switch to Postgres but I have
not been able to force Postgres to only use the index :-(
Any hints that may lead me back on track?
Thanks,
- Gummi
Re: [PERFORM] Index with all necessary columns - Postgres vs MSSQL
Hi,
Here are the answers to your questions:
1) I change the select statement so I am refering to 1 day at a time. In
that case the response time is similar. Basically, the data is not in
cache when I do that and the response time is about 23 seconds.
2) The list of IDs is provided by the middle layer and represents a logical
group.
btw: There are about 360 devices there. The distribution of dtStamp is
approx 200.000.000 rows / 360 devices / (4 months) which gives approx 4600
dtStamp values per device per day.
3) The query takes 23 sec vs 1 sec or lower in mssql.
We never update/delete and therefore the data is alway correct in the index
(never dirty). Therefore, Postgres could have used the data in it.
I started to add columns into indexes in Oracle for approx 15 years ago and
it was a brilliant discovery. This looks like a show stopper for me but I
will
Thanks,
- Gummi
On Wed, Feb 1, 2012 at 5:52 PM, Merlin Moncure wrote:
> On Wed, Feb 1, 2012 at 11:10 AM, Gudmundur Johannesson
> wrote:
> > Hi,
> >
> > I have a table in Postgres like:
> > CREATE TABLE test
> > (
> > id integer,
> > dtstamp timestamp without time zone,
> > rating real
> > )
> > CREATE INDEX test_all
> > ON test
> > USING btree
> > (id , dtstamp , rating);
> >
> > My db has around 200M rows and I have reduced my test select statement
> down
> > to:
> > SELECT count(1) FROM test
> > WHERE id in (58,83,88,98,124,141,170,195,202,252,265,293,305,331,348)
> > AND dtstamp between cast('2011-10-19 08:00:00' as timestamp) and
> > cast('2011-10-19 16:00:00' as timestamp)
> >
> > In Postgres this takes about 23 sec.
> > In MSSQL this takes about 1 sec.
> >
> > MSSQL only accesses the index and does not access the table it self (uses
> > only index scan)
> >
> > Postgres has the following plan:
> > "Aggregate (cost=130926.24..130926.25 rows=1 width=0)"
> > " -> Bitmap Heap Scan on test (cost=1298.97..130832.92 rows=37330
> > width=0)"
> > "Recheck Cond: ((id = ANY
> > ('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
> > AND (dtstamp >= '2011-10-19 08:00:00'::timestamp without time zone) AND
> > (dtstamp <= '2011-10-19 16:00:00'::timestamp without time zone))"
> > "-> Bitmap Index Scan on test_all (cost=0.00..1289.64
> rows=37330
> > width=0)"
> > " Index Cond: ((id = ANY
> > ('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
> > AND (dtstamp >= '2011-10-19 08:00:00'::timestamp without time zone) AND
> > (dtstamp <= '2011-10-19 16:00:00'::timestamp without time zone))"
> >
> > The results are disappointing since I want to switch to Postgres but I
> have
> > not been able to force Postgres to only use the index :-(
> >
> > Any hints that may lead me back on track?
>
> *) are the times in postgres stable across calls?
> *) where is the 'id list' coming from?
> *) how long does this query take?
>
> SELECT count(1) FROM test WHERE id = 202 AND AND dtstamp between
> '2011-10-19 08:00:00'::timestamp and '2011-10-19
> 16:00:00'::timestamp; ?
>
> The feature you're looking for in postgres is called 'index only
> scans' and an 9.2 will contain an implementation of that feature (see:
> http://rhaas.blogspot.com/2011/10/index-only-scans-weve-got-em.html).
>
> merlin
>
Re: [PERFORM] Index with all necessary columns - Postgres vs MSSQL
Hi,
I want to start by thanking you guys for a quick response and I will try to
provide all the information you request.
1) What version am I running:
"PostgreSQL 9.1.2, compiled by Visual C++ build 1500, 64-bit"
2) Schema:
CREATE TABLE test( id integer, dtstamp timestamp without time zone,
rating real) WITH ( OIDS=FALSE);
CREATE INDEX test_all ON test USING btree (id , dtstamp, rating);
200M rows
Table size 9833MB
Index size 7653 MB
3) Difference between the first and the second run time?
The statement executed is:
SELECT count(1) FROM test
WHERE id in (58,83,88,98,124,141,170,195,
202,252,265,293,305,331,348)
AND dtstamp between cast('2011-10-19 08:00:00' as timestamp) and
cast('2011-10-19 16:00:00' as timestamp)
a) 1st run = 26 seconds
b) 2nd run = 0.234 seconds
c) 3rd-6th run = 0.06 seconds
If I perform the query above for another day then I get 26 seconds for the
1st query.
4) What was the execution plan of it
"Aggregate (cost=151950.75..151950.76 rows=1 width=0)"
" -> Bitmap Heap Scan on data_cbm_reading cbm (cost=1503.69..151840.82
rows=43974 width=0)"
"Recheck Cond: ((virtual_id = ANY
('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
AND ("timestamp" >= '2011-10-19 08:00:00'::timestamp without time zone) AND
("timestamp" <= '2011-10-19 16:00:00'::timestamp without time zone))"
"-> Bitmap Index Scan on data_cbm_reading_all (cost=0.00..1492.70
rows=43974 width=0)"
" Index Cond: ((virtual_id = ANY
('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
AND ("timestamp" >= '2011-10-19 08:00:00'::timestamp without time zone) AND
("timestamp" <= '2011-10-19 16:00:00'::timestamp without time zone))"
5) In this case, I shut down the mssql server/machine and restart it. To
be on the safe side, I ensured the cache is empty using dbcc freeproccache
and dbcc dropcleanbuffers.
Then I tried the same statement as above:
a) 1st run = 0.8 seconds
b) 2nd, 3rd, ... run = 0.04 seconds
c) change the select statement for any another other day and run it again
give 1st run 0.5 seconds
d) 2nd, 3rd, ... run = 0.04 seconds
6) You wrote "I doubt covering indexes is going to make that query 23x
faster."
I decided to check out how mssql performs if it cannot use a covering
index. In order to do that, I drop my current index and create it again on
*id, dtstamp.* That forces mssql to look into the data file and the index
is no longer sufficient.
Running the following statement force the "rating" columns to be accessed:
select sum(rating)
FROM test
WHERE id in
(58,83,88,98,124,141,170,195,202,252,265,293,305,331,348)
AND dtstamp >= '2011-10-19 08:00:00' AND dtstamp <=
'2011-10-19 16:00:00'
a) 1st run = 20 seconds
b) 2nd run = 0.6
c) 3rd, ... run = 0.3 seconds
As you can see the response time gets just as bad as in Postgres.
Now lets recreate the mssql index with all the columns and double check the
response time:
a) 1st run = 2 seconds
b) 2nd run = 0.12
c) 3rd, ... run = 0.3 seconds
Therefore, I must conclude that in the case of mssql the "covering" index
is making a huge impact.
I have spent the whole day providing this data (takes a while to shuffle
200M rows) and tomorrow I will try your suggestion regarding two indexes.
*Do you think I should try using the latest build of the source for 9.2
since index-only-scan is "ready" according to
http://www.depesz.com/index.php/2011/10/08/waiting-for-9-2-index-only-scans/
?*
Thanks,
- Gummi
On Wed, Feb 1, 2012 at 7:35 PM, Merlin Moncure wrote:
> On Wed, Feb 1, 2012 at 12:50 PM, Gudmundur Johannesson
> wrote:
> > Here are the answers to your questions:
> > 1) I change the select statement so I am refering to 1 day at a time. In
> > that case the response time is similar. Basically, the data is not in
> cache
> > when I do that and the response time is about 23 seconds.
>
> what's the difference between the first and the second run time?
> Note, if you are only interested in the date the dtStamp falls on, you
> can exploit that in the index to knock 4 bytes off your index entry:
>
> CREATE INDEX test_all
> ON test
> USING btree
> (id , (dtstamp::date) , rating);
>
> and then use a similar expression to query it back out.
>
> > 3) The query takes 23 sec vs 1 sec or lower in mssql.
>
> I asked you to time a different query. Look again (and I'd like to
> see cached and uncached times).
>
> > We never update/delete and therefore the data is alway correct in the
> index
> > (never dirty). Therefore, Postgres could have used the data in it.
> >
> > I started to add columns into indexes in Oracle for approx 15 years ago
> and
> > it was a brilliant discovery. This looks like a show stopper for me but
> I
>
> I doubt covering indexes is going to make that query 23x faster.
> However, I bet we can get something worked out.
>
> merlin
>
Re: [PERFORM] Index with all necessary columns - Postgres vs MSSQL
May be I should first try to partition the table by date and see if that helps. Thanks, - Gummi On Thu, Feb 2, 2012 at 8:30 PM, Merlin Moncure wrote: > On Thu, Feb 2, 2012 at 10:41 AM, Gudmundur Johannesson > wrote: > > Do you think I should try using the latest build of the source for 9.2 > since > > index-only-scan is "ready" according to > > > http://www.depesz.com/index.php/2011/10/08/waiting-for-9-2-index-only-scans/ > > ? > > hm, interesting. > > You are simply welcome to try that and we would definitely like to see > your results. I looked around and didn't see any binaries for the > development snapshots for windows to test. That means you have to > compile postgres in order to test 9.2 at this point in time. Testing > and feedback of index only scan feature would be very much > appreciated. > > Generally speaking, postgresql source tree is very high quality -- > stuff should mostly work. The biggest annoyance is that you get lots > of catalog version bumps when pulling new versions of the sources > forcing a dump/reload. > > merlin >
Re: [PERFORM] Index with all necessary columns - Postgres vs MSSQL
On Tue, Feb 7, 2012 at 3:11 PM, Igor Neyman wrote:
> From: Gudmundur Johannesson [mailto:gudmundur.johannes...@gmail.com]
> Sent: Thursday, February 02, 2012 11:42 AM
> To: Merlin Moncure
> Cc: pgsql-performance@postgresql.org
> Subject: Re: Index with all necessary columns - Postgres vs MSSQL
>
> Hi,
>
> I want to start by thanking you guys for a quick response and I will try
> to provide all the information you request.
>
> 1) What version am I running:
> "PostgreSQL 9.1.2, compiled by Visual C++ build 1500, 64-bit"
>
> 2) Schema:
> CREATE TABLE test( id integer, dtstamp timestamp without time zone,
> rating real) WITH ( OIDS=FALSE);
> CREATE INDEX test_all ON test USING btree (id , dtstamp, rating);
> 200M rows
> Table size 9833MB
> Index size 7653 MB
>
> 3) Difference between the first and the second run time?
> The statement executed is:
> SELECT count(1) FROM test
> WHERE id in (58,83,88,98,124,141,170,195,
> 202,252,265,293,305,331,348)
> AND dtstamp between cast('2011-10-19 08:00:00' as timestamp) and
> cast('2011-10-19 16:00:00' as timestamp)
> a) 1st run = 26 seconds
> b) 2nd run = 0.234 seconds
> c) 3rd-6th run = 0.06 seconds
>
> If I perform the query above for another day then I get 26 seconds for the
> 1st query.
>
> 4) What was the execution plan of it
> "Aggregate (cost=151950.75..151950.76 rows=1 width=0)"
> " -> Bitmap Heap Scan on data_cbm_reading cbm (cost=1503.69..151840.82
> rows=43974 width=0)"
> "Recheck Cond: ((virtual_id = ANY
> ('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
> AND ("timestamp" >= '2011-10-19 08:00:00'::timestamp without time zone) AND
> ("timestamp" <= '2011-10-19 16:00:00'::timestamp without time zone))"
> "-> Bitmap Index Scan on data_cbm_reading_all
> (cost=0.00..1492.70 rows=43974 width=0)"
> " Index Cond: ((virtual_id = ANY
> ('{58,83,88,98,124,141,170,195,202,252,265,293,305,331,348}'::integer[]))
> AND ("timestamp" >= '2011-10-19 08:00:00'::timestamp without time zone) AND
> ("timestamp" <= '2011-10-19 16:00:00'::timestamp without time zone))"
>
> 5) In this case, I shut down the mssql server/machine and restart it. To
> be on the safe side, I ensured the cache is empty using dbcc freeproccache
> and dbcc dropcleanbuffers.
> Then I tried the same statement as above:
> a) 1st run = 0.8 seconds
> b) 2nd, 3rd, ... run = 0.04 seconds
> c) change the select statement for any another other day and run it again
> give 1st run 0.5 seconds
> d) 2nd, 3rd, ... run = 0.04 seconds
>
> 6) You wrote "I doubt covering indexes is going to make that query 23x
> faster."
> I decided to check out how mssql performs if it cannot use a covering
> index. In order to do that, I drop my current index and create it again on
> id, dtstamp. That forces mssql to look into the data file and the index is
> no longer sufficient.
> Running the following statement force the "rating" columns to be accessed:
> select sum(rating)
> FROM test
>WHERE id in
> (58,83,88,98,124,141,170,195,202,252,265,293,305,331,348)
>AND dtstamp >= '2011-10-19 08:00:00' AND dtstamp <=
> '2011-10-19 16:00:00'
> a) 1st run = 20 seconds
> b) 2nd run = 0.6
> c) 3rd, ... run = 0.3 seconds
> As you can see the response time gets just as bad as in Postgres.
> Now lets recreate the mssql index with all the columns and double check
> the response time:
> a) 1st run = 2 seconds
> b) 2nd run = 0.12
> c) 3rd, ... run = 0.3 seconds
>
>
> Therefore, I must conclude that in the case of mssql the "covering" index
> is making a huge impact.
>
> I have spent the whole day providing this data (takes a while to shuffle
> 200M rows) and tomorrow I will try your suggestion regarding two indexes.
>
> Do you think I should try using the latest build of the source for 9.2
> since index-only-scan is "ready" according to
> http://www.depesz.com/index.php/2011/10/08/waiting-for-9-2-index-only-scans/
> ?
>
> Thanks,
>- Gummi
>
>
> Gudmundur,
>
> Just for clarification purposes:
>
> This schema:
>
> CREATE TABLE test( id integer, dtstamp timestamp without time zone,
> rating real) WITH ( OIDS=FALSE);
> CREATE INDEX test_all ON test USING btree (id , dtstamp, rating);
>
> and this query plan:
>
> "Aggregate (cost=151950.75..151950.76 rows=1 width=0)"
> " -> Bitmap Heap Scan on data_cbm_reading cbm (cost=1503.69..151840.8
