3. hstore的GiST索引
hstore的GiST索引实现和全文检索的tsvector的GiST索引的实现差不过，都是签名文件索引。
3.1 GiST索引项的存储
1)所有索引项都采用签名向量压缩2)hstore中的所有键和所有值都进行哈希设置bit位得到一个签名向量
3)签名向量的大小为128bit
3.2 性能推论
1)索引尺寸比较小(因为签名向量的大小是固定的而且一般比原始数据小)2)每次查询要扫描的索引页很多（20%~40% 的 索引页）
3)每次键查询(?操作符)至少要recheck 1.6%(2/128)以上的数据行
每条记录包含的键值对越多,需要recheck的数据行越多
4)每次键值查询(@>操作符)至少要recheck 0.02%((2/128) * (2/128))以上的数据行
每条记录包含的键值对越多,需要recheck的数据行越多
假设每条记录平均包含10条键值对，需要recheck的数据行估算如下
键查询(?操作符)匹配：7.5%的数据行
点击(此处)折叠或打开
- testdb=# with recursive tx1(n,sign_bits) as
- testdb-# (select 1 n,1.0 sign_bits
- testdb(# union all
- testdb(# select n+1, sign_bits + 1 - sign_bits/128 from tx1 where n=10
- testdb(# )
- testdb-# select n,sign_bits,sign_bits/128 scan_probability from tx1 where n in(10);
- n | sign_bits | scan_probability
- ----+------------------------+------------------------
- 10 | 9.65566251563533320389 | 0.07543486340340104066
- (1 row)
键值查询(@>操作符)匹配：0.57%(0.57%=0.07543486340340104066*0.07543486340340104066)
但是这只是理论上的平均值，由于签名向量的冲突，个别查询的效率可能会很低。比如，如果大部分数据记录都包含某个key，而查询这个键值对的时候，碰巧它的值在签名向量中对应的bit位和这个很常用的key相同，那么大部分数据记录都要进行recheck。
3.3 相关代码
contrib/hstore/hstore_gist.c点击(此处)折叠或打开
- /* bigint defines */
- #define BITBYTE 8
- #define SIGLENINT 4 /* >122 => key will toast, so very */
- #define SIGLEN ( sizeof(int)*SIGLENINT )
- #define SIGLENBIT (SIGLEN*BITBYTE)
-
- typedef char BITVEC[SIGLEN];
- typedef char *BITVECP;
-
- ...
- Datum
- ghstore_compress(PG_FUNCTION_ARGS)
- {
- GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
- GISTENTRY *retval = entry;
-
- if (entry->leafkey)
- {
- GISTTYPE *res = (GISTTYPE *) palloc0(CALCGTSIZE(0));
- HStore *val = DatumGetHStoreP(entry->key);
- HEntry *hsent = ARRPTR(val);
- char *ptr = STRPTR(val);
- int count = HS_COUNT(val);
- int i;
-
- SET_VARSIZE(res, CALCGTSIZE(0));
-
- //遍历所有KV对
- for (i = 0; i count; ++i)
- {
- int h;
- //对key做哈希
- h = crc32_sz((char *) HS_KEY(hsent, ptr, i), HS_KEYLEN(hsent, i));
- HASH(GETSIGN(res), h);
- if (!HS_VALISNULL(hsent, i))
- {
- //对value做哈希
- h = crc32_sz((char *) HS_VAL(hsent, ptr, i), HS_VALLEN(hsent, i));
- HASH(GETSIGN(res), h);
- }
- }
-
- retval = (GISTENTRY *) palloc(sizeof(GISTENTRY));
- gistentryinit(*retval, PointerGetDatum(res),
- entry->rel, entry->page,
- entry->offset,
- FALSE);
- }
- else if (!ISALLTRUE(DatumGetPointer(entry->key)))
- {
- int32 i;
- GISTTYPE *res;
- BITVECP sign = GETSIGN(DatumGetPointer(entry->key));
-
- LOOPBYTE
- {
- if ((sign[i] & 0xff) != 0xff)
- PG_RETURN_POINTER(retval);
- }
-
- res = (GISTTYPE *) palloc(CALCGTSIZE(ALLISTRUE));
- SET_VARSIZE(res, CALCGTSIZE(ALLISTRUE));
- res->flag = ALLISTRUE;
-
- retval = (GISTENTRY *) palloc(sizeof(GISTENTRY));
- gistentryinit(*retval, PointerGetDatum(res),
- entry->rel, entry->page,
- entry->offset,
- FALSE);
- }
-
- PG_RETURN_POINTER(retval);
- }
3.4 性能验证
1)环境CentOS 6.5
PostgreSQL 9.4.0
2)测试数据准备
点击( 此处 )折叠或打开
- testdb=# create extension hstore;
- CREATE EXTENSION
- Time: 235.391 ms
- testdb=# create table tbhs(id serial,hs hstore);
- CREATE TABLE
- Time: 17.459 ms
- testdb=# insert into tbhs select id,hstore(id::text, md5(id::text)) from generate_series(1,100000) id;
- INSERT 0 100000
- Time: 970.613 ms
- testdb=# select pg_relation_size('tbhs');
- pg_relation_size
- ------------------
- 8445952
- (1 row)
-
- Time: 0.643 ms
- testdb=# select * from tbhs limit 5;
- id | hs
- ----+-----------------------------------------
- 1 | "1"=>"c4ca4238a0b923820dcc509a6f75849b"
- 2 | "2"=>"c81e728d9d4c2f636f067f89cc14862c"
- 3 | "3"=>"eccbc87e4b5ce2fe28308fd9f2a7baf3"
- 4 | "4"=>"a87ff679a2f3e71d9181a67b7542122c"
- 5 | "5"=>"e4da3b7fbbce2345d7772b0674a318d5"
- (5 rows)
-
- Time: 1.147 ms
2)无索引的查询
点击( 此处 )折叠或打开
- testdb=# explain (analyze,buffers) select * from tbhs where hs ? '10';
- QUERY PLAN
- -----------------------------------------------------------------------------------------------------
- Seq Scan on tbhs (cost=0.00..2281.00 rows=100 width=53) (actual time=0.025..31.587 rows=1 loops=1)
- Filter: (hs ? '10'::text)
- Rows Removed by Filter: 99999
- Buffers: shared hit=1031
- Planning time: 0.055 ms
- Execution time: 31.619 ms
- (6 rows)
-
- Time: 32.150 ms
3) 建立GiST索引再查询
点击(此处)折叠或打开
- testdb=# create index tbhs_idx_gist on tbhs using gist(hs);
- CREATE INDEX
- Time: 6243.003 ms
- testdb=# analyze tbhs;
- ANALYZE
- Time: 77.956 ms
- testdb=# select pg_relation_size('tbhs_idx_gist'),pg_relation_size('tbhs_idx_gist')/8192 index_pages;
- pg_relation_size | index_pages
- ------------------+-------------
- 4825088 | 589
- (1 row)
-
- Time: 0.456 ms
？查询扫描了45%（265/589）的索引页，Recheck了1.6%的数据行。
点击(此处)折叠或打开
- testdb=# explain (analyze,buffers) select * from tbhs where hs ? '10';
- QUERY PLAN
- ---------------------------------------------------------------------------------------------------------------------------
- Bitmap Heap Scan on tbhs (cost=5.06..302.42 rows=100 width=53) (actual time=3.003..4.637 rows=1 loops=1)
- Recheck Cond: (hs ? '10'::text)
- Rows Removed by Index Recheck: 1558
- Heap Blocks: exact=863
- Buffers: shared hit=1128
- -> Bitmap Index Scan on tbhs_idx_gist (cost=0.00..5.03 rows=100 width=0) (actual time=2.869..2.869 rows=1559 loops=1)
- Index Cond: (hs ? '10'::text)
- Buffers: shared hit=265
- Planning time: 0.153 ms
- Execution time: 5.073 ms
- (10 rows)
-
- Time: 6.209 ms
@>查询扫描了19%（114/589）的索引页，Recheck了0.012%的数据行。
点击(此处)折叠或打开
- testdb=# explain (analyze,buffers) select * from tbhs where hs @> '1=>c4ca4238a0b923820dcc509a6f75849b';
- QUERY PLAN
- -------------------------------------------------------------------------------------------------------------------------
- Bitmap Heap Scan on tbhs (cost=5.06..302.42 rows=100 width=53) (actual time=2.123..2.205 rows=1 loops=1)
- Recheck Cond: (hs @> '"1"=>"c4ca4238a0b923820dcc509a6f75849b"'::hstore)
- Rows Removed by Index Recheck: 11
- Heap Blocks: exact=12
- Buffers: shared hit=126
- -> Bitmap Index Scan on tbhs_idx_gist (cost=0.00..5.03 rows=100 width=0) (actual time=2.097..2.097 rows=12 loops=1)
- Index Cond: (hs @> '"1"=>"c4ca4238a0b923820dcc509a6f75849b"'::hstore)
- Buffers: shared hit=114
- Planning time: 0.407 ms
- Execution time: 2.294 ms
- (10 rows)
-
- Time: 3.332 ms
4)建立GIN索引再查询
建立GIN索引对比一下。
点击(此处)折叠或打开
- testdb=# drop index tbhs_idx_gist;
- DROP INDEX
- Time: 7.036 ms
- testdb=# create index tbhs_idx_gin on tbhs using gin(hs);
- CREATE INDEX
- Time: 2244.241 ms
- testdb=# analyze tbhs;
- ANALYZE
- Time: 64.608 ms
- testdb=# select pg_relation_size('tbhs_idx_gin'),pg_relation_size('tbhs_idx_gin')/8192 index_pages;
- pg_relation_size | index_pages
- ------------------+-------------
- 17629184 | 2152
- (1 row)
-
- Time: 0.641 ms
- testdb=# explain (analyze,buffers) select * from tbhs where hs ? '10';
- QUERY PLAN
- ------------------------------------------------------------------------------------------------------------------------
- Bitmap Heap Scan on tbhs (cost=16.77..314.14 rows=100 width=53) (actual time=0.096..0.097 rows=1 loops=1)
- Recheck Cond: (hs ? '10'::text)
- Heap Blocks: exact=1
- Buffers: shared hit=5
- -> Bitmap Index Scan on tbhs_idx_gin (cost=0.00..16.75 rows=100 width=0) (actual time=0.086..0.086 rows=1 loops=1)
- Index Cond: (hs ? '10'::text)
- Buffers: shared hit=4
- Planning time: 0.349 ms
- Execution time: 0.144 ms
- (9 rows)
-
- Time: 1.014 ms
- testdb=# explain (analyze,buffers) select * from tbhs where hs @> '1=>c4ca4238a0b923820dcc509a6f75849b';
- QUERY PLAN
- ------------------------------------------------------------------------------------------------------------------------
- Bitmap Heap Scan on tbhs (cost=28.77..326.14 rows=100 width=53) (actual time=0.070..0.071 rows=1 loops=1)
- Recheck Cond: (hs @> '"1"=>"c4ca4238a0b923820dcc509a6f75849b"'::hstore)
- Heap Blocks: exact=1
- Buffers: shared hit=8
- -> Bitmap Index Scan on tbhs_idx_gin (cost=0.00..28.75 rows=100 width=0) (actual time=0.047..0.047 rows=1 loops=1)
- Index Cond: (hs @> '"1"=>"c4ca4238a0b923820dcc509a6f75849b"'::hstore)
- Buffers: shared hit=7
- Planning time: 0.131 ms
- Execution time: 0.105 ms
- (9 rows)
-
- Time: 0.661 ms
3.5 参考
http://blog.chinaunix.net/uid-20726500-id-4884626.htmlhttp://blog.chinaunix.net/uid-20726500-id-4884681.html
http://blog.chinaunix.net/uid-20726500-id-4886571.html
http://blog.chinaunix.net/uid-259788-id-4279627.html
