From cfd4804b1ebed8108ef6caedc76fb6eb37bc9724 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 7 Jun 2016 12:19:18 -0400
Subject: [PATCH] Improve documentation for contrib/bloom.
Michael Paquier, David Johnston, Tom Lane
Discussion: <CAB7nPqQB8dcFmY1uodmiJOSZdhBFOx-us-uW6rfYrzhpEiBR2g@mail.gmail.com>
---
doc/src/sgml/bloom.sgml | 243 ++++++++++++++++++++++++----------------
1 file changed, 144 insertions(+), 99 deletions(-)
diff --git a/doc/src/sgml/bloom.sgml b/doc/src/sgml/bloom.sgml
index 8667763c43c..b3d0665deb3 100644
--- a/doc/src/sgml/bloom.sgml
+++ b/doc/src/sgml/bloom.sgml
@@ -8,43 +8,42 @@
</indexterm>
<para>
- <literal>bloom</> is a module that implements an index access method. It comes
- as an example of custom access methods and generic WAL record usage. But it
- is also useful in itself.
+ <literal>bloom</> provides an index access method based on
+ <ulink url="http://en.wikipedia.org/wiki/Bloom_filter">Bloom filters</ulink>.
</para>
- <sect2>
- <title>Introduction</title>
+ <para>
+ A Bloom filter is a space-efficient data structure that is used to test
+ whether an element is a member of a set. In the case of an index access
+ method, it allows fast exclusion of non-matching tuples via signatures
+ whose size is determined at index creation.
+ </para>
- <para>
- The implementation of a
- <ulink url="http://en.wikipedia.org/wiki/Bloom_filter">Bloom filter</ulink>
- allows fast exclusion of non-candidate tuples via signatures.
- Since a signature is a lossy representation of all indexed attributes,
- search results must be rechecked using heap information.
- The user can specify signature length in bits (default 80, maximum 4096)
- and the number of bits generated for each index column (default 2,
- maximum 4095).
- </para>
+ <para>
+ A signature is a lossy representation of the indexed attribute(s), and as
+ such is prone to reporting false positives; that is, it may be reported
+ that an element is in the set, when it is not. So index search results
+ must always be rechecked using the actual attribute values from the heap
+ entry. Larger signatures reduce the odds of a false positive and thus
+ reduce the number of useless heap visits, but of course also make the index
+ larger and hence slower to scan.
+ </para>
- <para>
- This index is useful if a table has many attributes and queries include
- arbitrary combinations of them. A traditional <literal>btree</> index is
- faster than a bloom index, but it can require many indexes to support all
- possible queries where one needs only a single bloom index. A Bloom index
- supports only equality comparison. Since it's a signature file, and not a
- tree, it always must be read fully, but sequentially, so that index search
- performance is constant and doesn't depend on a query.
- </para>
- </sect2>
+ <para>
+ This type of index is most useful when a table has many attributes and
+ queries test arbitrary combinations of them. A traditional btree index is
+ faster than a bloom index, but it can require many btree indexes to support
+ all possible queries where one needs only a single bloom index. Note
+ however that bloom indexes only support equality queries, whereas btree
+ indexes can also perform inequality and range searches.
+ </para>
<sect2>
<title>Parameters</title>
<para>
- <literal>bloom</> indexes accept the following parameters in the
- <literal>WITH</>
- clause.
+ A <literal>bloom</> index accepts the following parameters in its
+ <literal>WITH</> clause:
</para>
<variablelist>
@@ -52,7 +51,8 @@
<term><literal>length</></term>
<listitem>
<para>
- Length of signature in bits
+ Length of each signature (index entry) in bits. The default
+ is <literal>80</> bits and maximum is <literal>4096</>.
</para>
</listitem>
</varlistentry>
@@ -62,7 +62,10 @@
<term><literal>col1 — col32</></term>
<listitem>
<para>
- Number of bits generated for each index column
+ Number of bits generated for each index column. Each parameter's name
+ refers to the number of the index column that it controls. The default
+ is <literal>2</> bits and maximum is <literal>4095</>. Parameters for
+ index columns not actually used are ignored.
</para>
</listitem>
</varlistentry>
@@ -73,7 +76,7 @@
<title>Examples</title>
<para>
- An example of an index definition is given below.
+ This is an example of creating a bloom index:
</para>
<programlisting>
@@ -82,92 +85,135 @@ CREATE INDEX bloomidx ON tbloom USING bloom (i1,i2,i3)
</programlisting>
<para>
- Here, we created a bloom index with a signature length of 80 bits,
- and attributes i1 and i2 mapped to 2 bits, and attribute i3 mapped to 4 bits.
+ The index is created with a signature length of 80 bits, with attributes
+ i1 and i2 mapped to 2 bits, and attribute i3 mapped to 4 bits. We could
+ have omitted the <literal>length</>, <literal>col1</>,
+ and <literal>col2</> specifications since those have the default values.
</para>
<para>
- Here is a fuller example of index definition and usage:
+ Here is a more complete example of bloom index definition and usage, as
+ well as a comparison with equivalent btree indexes. The bloom index is
+ considerably smaller than the btree index, and can perform better.
</para>
<programlisting>
-CREATE TABLE tbloom AS
-SELECT
- random()::int as i1,
- random()::int as i2,
- random()::int as i3,
- random()::int as i4,
- random()::int as i5,
- random()::int as i6,
- random()::int as i7,
- random()::int as i8,
- random()::int as i9,
- random()::int as i10,
- random()::int as i11,
- random()::int as i12,
- random()::int as i13
-FROM
- generate_series(1,1000);
-CREATE INDEX bloomidx ON tbloom USING
- bloom (i1, i2, i3, i4, i5, i6, i7, i8, i9, i10, i11, i12);
-SELECT pg_relation_size('bloomidx');
-CREATE index btree_idx ON tbloom(i1,i2,i3,i4,i5,i6,i7,i8,i9,i10,i11,i12);
-SELECT pg_relation_size('btree_idx');
+=# CREATE TABLE tbloom AS
+ SELECT
+ (random() * 1000000)::int as i1,
+ (random() * 1000000)::int as i2,
+ (random() * 1000000)::int as i3,
+ (random() * 1000000)::int as i4,
+ (random() * 1000000)::int as i5,
+ (random() * 1000000)::int as i6
+ 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)
</programlisting>
+ <para>
+ A sequential scan over this large table takes a long time:
<programlisting>
-=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 20 AND i10 = 15;
- QUERY PLAN
------------------------------------------------------------------------------------------------------------------
- Bitmap Heap Scan on tbloom (cost=1.50..5.52 rows=1 width=52) (actual time=0.057..0.057 rows=0 loops=1)
- Recheck Cond: ((i2 = 20) AND (i10 = 15))
- -> Bitmap Index Scan on bloomidx (cost=0.00..1.50 rows=1 width=0) (actual time=0.041..0.041 rows=9 loops=1)
- Index Cond: ((i2 = 20) AND (i10 = 15))
- Total runtime: 0.081 ms
+=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
+ QUERY PLAN
+------------------------------------------------------------------------------------------------------------
+ Seq Scan on tbloom (cost=0.00..213694.08 rows=1 width=24) (actual time=1445.438..1445.438 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
(5 rows)
</programlisting>
+ </para>
<para>
- Seqscan is slow.
+ So the planner will usually select an index scan if possible.
+ With a btree index, we get results like this:
+<programlisting>
+=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
+ QUERY PLAN
+--------------------------------------------------------------------------------------------------------------------------------
+ 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
+(5 rows)
+</programlisting>
</para>
+ <para>
+ Bloom is better than btree in handling this type of search:
<programlisting>
-=# SET enable_bitmapscan = off;
-=# SET enable_indexscan = off;
-=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 20 AND i10 = 15;
- QUERY PLAN
---------------------------------------------------------------------------------------------------
- Seq Scan on tbloom (cost=0.00..25.00 rows=1 width=52) (actual time=0.162..0.162 rows=0 loops=1)
- Filter: ((i2 = 20) AND (i10 = 15))
- Total runtime: 0.181 ms
-(3 rows)
+=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
+ QUERY PLAN
+---------------------------------------------------------------------------------------------------------------------------
+ Bitmap Heap Scan on tbloom (cost=178435.39..178439.41 rows=1 width=24) (actual time=76.698..76.698 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)
+ Index Cond: ((i2 = 898732) AND (i5 = 123451))
+ Planning time: 0.475 ms
+ Execution time: 76.778 ms
+(8 rows)
</programlisting>
+ 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 <literal>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).
+ </para>
- <para>
- A btree index will be not used for this query.
- </para>
-
+ <para>
+ Now, the main problem with the btree search is that btree is inefficient
+ when the search conditions do not constrain the leading index column(s).
+ A better strategy for btree is to create a separate index on each column.
+ Then the planner will choose something like this:
<programlisting>
-=# DROP INDEX bloomidx;
-=# CREATE INDEX btree_idx ON tbloom(i1, i2, i3, i4, i5, i6, i7, i8, i9, i10, i11, i12);
-=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 20 AND i10 = 15;
- QUERY PLAN
---------------------------------------------------------------------------------------------------
- Seq Scan on tbloom (cost=0.00..25.00 rows=1 width=52) (actual time=0.210..0.210 rows=0 loops=1)
- Filter: ((i2 = 20) AND (i10 = 15))
- Total runtime: 0.250 ms
-(3 rows)
+=# EXPLAIN ANALYZE SELECT * FROM tbloom WHERE i2 = 898732 AND i5 = 123451;
+ QUERY PLAN
+------------------------------------------------------------------------------------------------------------------------------
+ Bitmap Heap Scan on tbloom (cost=9.29..13.30 rows=1 width=24) (actual time=0.148..0.148 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)
+ 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)
+ Index Cond: (i2 = 898732)
+ Planning time: 2.049 ms
+ Execution time: 0.280 ms
+(9 rows)
</programlisting>
+ 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.
+ </para>
</sect2>
<sect2>
- <title>Opclass interface</title>
+ <title>Operator Class Interface</title>
<para>
- The Bloom opclass interface is simple. It requires 1 supporting function:
- a hash function for the indexing datatype. It provides 1 search operator:
- the equality operator. The example below shows <literal>opclass</>
- definition for <literal>text</> datatype.
+ An operator class for bloom indexes requires only a hash function for the
+ indexed datatype and an equality operator for searching. This example
+ shows the opclass definition for the <type>text</> data type:
</para>
<programlisting>
@@ -179,22 +225,21 @@ DEFAULT FOR TYPE text USING bloom AS
</sect2>
<sect2>
- <title>Limitation</title>
+ <title>Limitations</title>
<para>
-
<itemizedlist>
<listitem>
<para>
- For now, only opclasses for <literal>int4</>, <literal>text</> come
- with the module. However, users may define more of them.
+ Only operator classes for <type>int4</> and <type>text</> are
+ included with the module.
</para>
</listitem>
<listitem>
<para>
- Only the <literal>=</literal> operator is supported for search at the
- moment. But it's possible to add support for arrays with contains and
- intersection operations in the future.
+ Only the <literal>=</literal> operator is supported for search. But
+ it is possible to add support for arrays with union and intersection
+ operations in the future.
</para>
</listitem>
</itemizedlist>
--
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