diff --git a/doc/src/sgml/bloom.sgml b/doc/src/sgml/bloom.sgml
index 285b67b3f1..d1cf9ac24a 100644
--- a/doc/src/sgml/bloom.sgml
+++ b/doc/src/sgml/bloom.sgml
@@ -110,75 +110,70 @@ CREATE INDEX bloomidx ON tbloom USING bloom (i1,i2,i3)
FROM
generate_series(1,10000000);
SELECT 10000000
-=# CREATE INDEX bloomidx ON tbloom USING bloom (i1, i2, i3, i4, i5, i6);
-CREATE INDEX
-=# SELECT pg_size_pretty(pg_relation_size('bloomidx'));
- pg_size_pretty
-----------------
- 153 MB
-(1 row)
-=# CREATE index btreeidx ON tbloom (i1, i2, i3, i4, i5, i6);
-CREATE INDEX
-=# SELECT pg_size_pretty(pg_relation_size('btreeidx'));
- pg_size_pretty
-----------------
- 387 MB
-(1 row)
A sequential scan over this large table takes a long time:
=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
- QUERY PLAN
--------------------------------------------------------------------&zwsp;-----------------------------------------
- Seq Scan on tbloom (cost=0.00..213694.08 rows=1 width=24) (actual time=1445.438..1445.438 rows=0 loops=1)
+ QUERY PLAN
+-------------------------------------------------------------------&zwsp;-----------------------------------
+ Seq Scan on tbloom (cost=0.00..2137.14 rows=3 width=24) (actual time=16.971..16.971 rows=0 loops=1)
Filter: ((i2 = 898732) AND (i5 = 123451))
- Rows Removed by Filter: 10000000
- Planning time: 0.177 ms
- Execution time: 1445.473 ms
+ Rows Removed by Filter: 100000
+ Planning Time: 0.346 ms
+ Execution Time: 16.988 ms
(5 rows)
- So the planner will usually select an index scan if possible.
- With a btree index, we get results like this:
+ Even with the btree index defined the result will still be a
+ sequential scan:
+=# CREATE INDEX btreeidx ON tbloom (i1, i2, i3, i4, i5, i6);
+CREATE INDEX
+=# SELECT pg_size_pretty(pg_relation_size('btreeidx'));
+ pg_size_pretty
+----------------
+ 3976 kB
+(1 row)
=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
- QUERY PLAN
--------------------------------------------------------------------&zwsp;-------------------------------------------------------------
- Index Only Scan using btreeidx on tbloom (cost=0.56..298311.96 rows=1 width=24) (actual time=445.709..445.709 rows=0 loops=1)
- Index Cond: ((i2 = 898732) AND (i5 = 123451))
- Heap Fetches: 0
- Planning time: 0.193 ms
- Execution time: 445.770 ms
+ QUERY PLAN
+-------------------------------------------------------------------&zwsp;-----------------------------------
+ Seq Scan on tbloom (cost=0.00..2137.00 rows=2 width=24) (actual time=12.805..12.805 rows=0 loops=1)
+ Filter: ((i2 = 898732) AND (i5 = 123451))
+ Rows Removed by Filter: 100000
+ Planning Time: 0.138 ms
+ Execution Time: 12.817 ms
(5 rows)
- Bloom is better than btree in handling this type of search:
+ Having the bloom index defined on the table is better than btree in
+ handling this type of search:
+=# CREATE INDEX bloomidx ON tbloom USING bloom (i1, i2, i3, i4, i5, i6);
+CREATE INDEX
+=# SELECT pg_size_pretty(pg_relation_size('bloomidx'));
+ pg_size_pretty
+----------------
+ 1584 kB
+(1 row)
=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
- QUERY PLAN
--------------------------------------------------------------------&zwsp;--------------------------------------------------------
- Bitmap Heap Scan on tbloom (cost=178435.39..178439.41 rows=1 width=24) (actual time=76.698..76.698 rows=0 loops=1)
+ QUERY PLAN
+-------------------------------------------------------------------&zwsp;--------------------------------------------------
+ Bitmap Heap Scan on tbloom (cost=1792.00..1799.69 rows=2 width=24) (actual time=0.388..0.388 rows=0 loops=1)
Recheck Cond: ((i2 = 898732) AND (i5 = 123451))
- Rows Removed by Index Recheck: 2439
- Heap Blocks: exact=2408
- -> Bitmap Index Scan on bloomidx (cost=0.00..178435.39 rows=1 width=0) (actual time=72.455..72.455 rows=2439 loops=1)
+ Rows Removed by Index Recheck: 29
+ Heap Blocks: exact=28
+ -> Bitmap Index Scan on bloomidx (cost=0.00..1792.00 rows=2 width=0) (actual time=0.356..0.356 rows=29 loops=1)
Index Cond: ((i2 = 898732) AND (i5 = 123451))
- Planning time: 0.475 ms
- Execution time: 76.778 ms
+ Planning Time: 0.099 ms
+ Execution Time: 0.408 ms
(8 rows)
- Note the relatively large number of false positives: 2439 rows were
- selected to be visited in the heap, but none actually matched the
- query. We could reduce that by specifying a larger signature length.
- In this example, creating the index with length=200
- reduced the number of false positives to 55; but it doubled the index size
- (to 306 MB) and ended up being slower for this query (125 ms overall).
@@ -187,24 +182,36 @@ CREATE INDEX
A better strategy for btree is to create a separate index on each column.
Then the planner will choose something like this:
+=# CREATE INDEX btreeidx1 ON tbloom (i1);
+CREATE INDEX
+=# CREATE INDEX btreeidx2 ON tbloom (i2);
+CREATE INDEX
+=# CREATE INDEX btreeidx3 ON tbloom (i3);
+CREATE INDEX
+=# CREATE INDEX btreeidx4 ON tbloom (i4);
+CREATE INDEX
+=# CREATE INDEX btreeidx5 ON tbloom (i5);
+CREATE INDEX
+=# CREATE INDEX btreeidx6 ON tbloom (i6);
+CREATE INDEX
=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
- QUERY PLAN
--------------------------------------------------------------------&zwsp;-----------------------------------------------------------
- Bitmap Heap Scan on tbloom (cost=9.29..13.30 rows=1 width=24) (actual time=0.148..0.148 rows=0 loops=1)
+ QUERY PLAN
+-------------------------------------------------------------------&zwsp;--------------------------------------------------------
+ Bitmap Heap Scan on tbloom (cost=24.34..32.03 rows=2 width=24) (actual time=0.028..0.029 rows=0 loops=1)
Recheck Cond: ((i5 = 123451) AND (i2 = 898732))
- -> BitmapAnd (cost=9.29..9.29 rows=1 width=0) (actual time=0.145..0.145 rows=0 loops=1)
- -> Bitmap Index Scan on tbloom_i5_idx (cost=0.00..4.52 rows=11 width=0) (actual time=0.089..0.089 rows=10 loops=1)
+ -> BitmapAnd (cost=24.34..24.34 rows=2 width=0) (actual time=0.027..0.027 rows=0 loops=1)
+ -> Bitmap Index Scan on btreeidx5 (cost=0.00..12.04 rows=500 width=0) (actual time=0.026..0.026 rows=0 loops=1)
Index Cond: (i5 = 123451)
- -> Bitmap Index Scan on tbloom_i2_idx (cost=0.00..4.52 rows=11 width=0) (actual time=0.048..0.048 rows=8 loops=1)
+ -> Bitmap Index Scan on btreeidx2 (cost=0.00..12.04 rows=500 width=0) (never executed)
Index Cond: (i2 = 898732)
- Planning time: 2.049 ms
- Execution time: 0.280 ms
+ Planning Time: 0.491 ms
+ Execution Time: 0.055 ms
(9 rows)
Although this query runs much faster than with either of the single
- indexes, we pay a large penalty in index size. Each of the single-column
- btree indexes occupies 214 MB, so the total space needed is over 1.2GB,
- more than 8 times the space used by the bloom index.
+ indexes, we pay a penalty in index size. Each of the single-column
+ btree indexes occupies 2 MB, so the total space needed is 12 MB,
+ eight times the space used by the bloom index.