performance of bitmap scans in nested loop joins

["Sergey E. Koposov" <math(at)sai(dot)msu(dot)ru>]

Hello All,

I'd like to report about suprising (for me) results of performance testing of
bitmap  indexes in nested loop join plans. 

I have the table q3c

test=# \d q3c
     Table "public.q3c"
 Column |  Type  | Modifiers 
--------+--------+-----------
 ipix   | bigint | 
 errbox | box    | 
 ra     | real   | 
 dec    | real   | 
Indexes:
    "ipix_idx" UNIQUE, btree (ipix)

####################

test=# SELECT count(*) from q3c;
  count  
---------
 3000000
(1 row)


First, two join plans with just simple index scan: 

############# 1) 

test=# EXPLAIN ANALYZE SELECT * FROM q3c,q3c AS q3cs WHERE 
q3c.ipix>=q3cs.ipix-3 AND q3c.ipix<=q3cs.ipix+3;
                                                            QUERY PLAN                                                             
-----------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.01..49059519577.21 rows=1000010666695 width=96) (actual time=0.137..39385.725 rows=3000000 loops=1)
   ->  Seq Scan on q3c q3cs  (cost=0.00..60928.16 rows=3000016 width=48) (actual time=0.007..3659.171 rows=3000000 loops=1)
   ->  Index Scan using ipix_idx on q3c  (cost=0.01..9686.37 rows=333335 width=48) (actual time=0.007..0.009 rows=1 loops=3000000)
         Index Cond: ((q3c.ipix >= ("outer".ipix - 3)) AND (q3c.ipix <= ("outer".ipix + 3)))
 Total runtime: 40755.390 ms
(5 rows)


############# 2)

test=# EXPLAIN ANALYZE SELECT * FROM q3c,q3c as q3cs WHERE 
(q3c.ipix>=q3cs.ipix-1000 AND q3c.ipix<=q3cs.ipix-993);
                                                            QUERY PLAN                                                             
-----------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.01..49059519577.21 rows=1000010666695 width=96) (actual time=28058.983..28058.983 rows=0 loops=1)
   ->  Seq Scan on q3c q3cs  (cost=0.00..60928.16 rows=3000016 width=48) (actual time=0.061..3803.598 rows=3000000 loops=1)
   ->  Index Scan using ipix_idx on q3c  (cost=0.01..9686.37 rows=333335 width=48) (actual time=0.006..0.006 rows=0 loops=3000000)
         Index Cond: ((q3c.ipix >= ("outer".ipix - 1000)) AND (q3c.ipix <= ("outer".ipix - 993)))
 Total runtime: 28059.066 ms
(5 rows)

#############

And now I combine them in one:


test=# EXPLAIN ANALYZE SELECT * FROM q3c,q3c AS q3cs WHERE 
(q3c.ipix>=q3cs.ipix-3 AND q3c.ipix<=q3cs.ipix+3) OR 
(q3c.ipix>=q3cs.ipix-1000 AND q3c.ipix<=q3cs.ipix-993);
                                                                                   QUERY PLAN                                                                                    
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=5832.03..190281569757.46 rows=1888909037091 width=96) (actual time=0.180..122444.610 rows=3000000 loops=1)
   ->  Seq Scan on q3c q3cs  (cost=0.00..60928.16 rows=3000016 width=48) (actual time=0.063..3871.731 rows=3000000 loops=1)
   ->  Bitmap Heap Scan on q3c  (cost=5832.03..43426.73 rows=666670 width=48) (actual time=0.033..0.034 rows=1 loops=3000000)
         Recheck Cond: (((q3c.ipix >= ("outer".ipix - 3)) AND (q3c.ipix <= ("outer".ipix + 3))) OR ((q3c.ipix >= ("outer".ipix - 1000)) AND (q3c.ipix <= ("outer".ipix - 993))))
         ->  BitmapOr  (cost=5832.03..5832.03 rows=666670 width=0) (actual time=0.029..0.029 rows=0 loops=3000000)
               ->  Bitmap Index Scan on ipix_idx  (cost=0.00..2916.02 rows=333335 width=0) (actual time=0.014..0.014 rows=1 loops=3000000)
                     Index Cond: ((q3c.ipix >= ("outer".ipix - 3)) AND (q3c.ipix <= ("outer".ipix + 3)))
               ->  Bitmap Index Scan on ipix_idx  (cost=0.00..2916.02 rows=333335 width=0) (actual time=0.011..0.011 rows=0 loops=3000000)
                     Index Cond: ((q3c.ipix >= ("outer".ipix - 1000)) AND (q3c.ipix <= ("outer".ipix - 993)))
 Total runtime: 124366.545 ms
(10 rows)


So, we see that total time of the plan with bitmap scan is roughly two times
greater than the sum of times of individual index scans.  


I see that in my case even simple union is significantly faster than bitmap
indexes. 

test=# EXPLAIN ANALYZE SELECT * FROM q3c,q3c AS q3cs WHERE 
(q3c.ipix>=q3cs.ipix-3 AND q3c.ipix<=q3cs.ipix+3) 
UNION ALL
SELECT * FROM q3c,q3c AS q3cs WHERE 
(q3c.ipix>=q3cs.ipix-1000 AND q3c.ipix<=q3cs.ipix-993);
                                                                  QUERY PLAN                                                                   
-----------------------------------------------------------------------------------------------------------------------------------------------
 Append  (cost=0.01..118119252488.32 rows=2000021333390 width=96) (actual time=0.139..75048.897 rows=3000000 loops=1)
   ->  Subquery Scan "*SELECT* 1"  (cost=0.01..59059626244.16 rows=1000010666695 width=96) (actual time=0.139..44221.409 rows=3000000 loops=1)
         ->  Nested Loop  (cost=0.01..49059519577.21 rows=1000010666695 width=96) (actual time=0.137..40735.906 rows=3000000 loops=1)
               ->  Seq Scan on q3c q3cs  (cost=0.00..60928.16 rows=3000016 width=48) (actual time=0.063..3719.637 rows=3000000 loops=1)
               ->  Index Scan using ipix_idx on q3c  (cost=0.01..9686.37 rows=333335 width=48) (actual time=0.007..0.009 rows=1 loops=3000000)
                     Index Cond: ((q3c.ipix >= ("outer".ipix - 3)) AND (q3c.ipix <= ("outer".ipix + 3)))
   ->  Subquery Scan "*SELECT* 2"  (cost=0.01..59059626244.16 rows=1000010666695 width=96) (actual time=28120.449..28120.449 rows=0 loops=1)
         ->  Nested Loop  (cost=0.01..49059519577.21 rows=1000010666695 width=96) (actual time=28120.447..28120.447 rows=0 loops=1)
               ->  Seq Scan on q3c q3cs  (cost=0.00..60928.16 rows=3000016 width=48) (actual time=0.023..3649.739 rows=3000000 loops=1)
               ->  Index Scan using ipix_idx on q3c  (cost=0.01..9686.37 rows=333335 width=48) (actual time=0.006..0.006 rows=0 loops=3000000)
                     Index Cond: ((q3c.ipix >= ("outer".ipix - 1000)) AND (q3c.ipix <= ("outer".ipix - 993)))
 Total runtime: 76413.737 ms
(12 rows)


Last note: all those queries were run in fully cached regime on P4 2.8Ghz. 
I used the yesterday's CVS snapshot.

Are those performance results expected for the bitmap index scans ?  

Thanks in advance for any comments.


With Best Regards,
		Sergey Koposov

------------------------------------------------------------
Sergey E. Koposov
Sternberg Astronomical Institute, Moscow University (Russia)
Max-Planck Institute for Astronomy (Germany) 
Internet: math(at)sai(dot)msu(dot)ru, http://lnfm1.sai.msu.su/~math/