Re: Index with all necessary columns - Postgres vs MSSQL

From: Gudmundur Johannesson [mailto:gudmundur(dot)johannesson(at)gmail(dot)com]
Sent: Thursday, February 02, 2012 11:42 AM
To: Merlin Moncure
Cc: pgsql-performance(at)postgresql(dot)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..151840.82 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))"

reference different table and index names.
Also, EXPLAIN ANALYZE would provide additional info compared to just EXPLAIN.

One option you could try, is to cluster your table based on " test_all" index, and see if it makes a difference.
BTW., in SQL Server your "covering" index - is it clustered?

Regards,
Igor Neyman