本篇内容介绍了“PostgreSQL统计信息中相关的数据字典表有哪些”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！

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一、pg_class

在pg_class数据字典表中,存储了Relation(包括Index)的两个统计信息:页面占用总数relpages和元组总数reltuples,在优化器执行物理优化时用于估算访问路径的启动成本和总成本.
测试脚本如下,在t_grxx表中插入10w数据,创建2个索引:

drop table if exists t_grxx;
create table t_grxx(dwbh varchar(10),grbh varchar(10),xm varchar(20),xb varchar(10),nl int);

insert into t_grxx(dwbh,grbh,xm,xb,nl) 
select generate_series(1,100000)/10||'',generate_series(1,100000),'XM'||generate_series(1,100000),
(case when (floor(random()*2)=0) then '男' else '女' end),floor(random() * 100 + 1)::int;

create index idx_t_grxx_grbh on t_grxx(grbh);
create index idx_t_grxx_dwbh on t_grxx(dwbh);

基础关系t_grxx中的统计信息

testdb=# select relpages,reltuples from pg_class where relname = 't_grxx';
 relpages | reltuples 
----------+-----------
      726 |    100000 --> 页面数726,元组数10,000
(1 row)

索引idx_t_grxx_grbh的统计信息

testdb=# select relpages,reltuples from pg_class where relname = 'idx_t_grxx_grbh';
 relpages | reltuples 
----------+-----------
      276 |    100000 --> 页面数276,元组数10,000
(1 row)

二、pg_statistic

pg_statistic是PG中存储统计信息的主要数据字典表.通过命令ANALYZE生成统计数据,这些统计数据在计划阶段提供给优化器使用,是成本估算的基础.
pg_statistic的表结构如下:

testdb=# \d pg_statistic
             Table "pg_catalog.pg_statistic"
   Column    |   Type   | Collation | Nullable | Default 
-------------+----------+-----------+----------+---------
 starelid    | oid      |           | not null | 
 staattnum   | smallint |           | not null | 
 stainherit  | boolean  |           | not null | 
 stanullfrac | real     |           | not null | 
 stawidth    | integer  |           | not null | 
 stadistinct | real     |           | not null | 
 stakind1    | smallint |           | not null | 
 stakind2    | smallint |           | not null | 
 stakind3    | smallint |           | not null | 
 stakind4    | smallint |           | not null | 
 stakind5    | smallint |           | not null | 
 staop1      | oid      |           | not null | 
 staop2      | oid      |           | not null | 
 staop3      | oid      |           | not null | 
 staop4      | oid      |           | not null | 
 staop5      | oid      |           | not null | 
 stanumbers1 | real[]   |           |          | 
 stanumbers2 | real[]   |           |          | 
 stanumbers3 | real[]   |           |          | 
 stanumbers4 | real[]   |           |          | 
 stanumbers5 | real[]   |           |          | 
 stavalues1  | anyarray |           |          | 
 stavalues2  | anyarray |           |          | 
 stavalues3  | anyarray |           |          | 
 stavalues4  | anyarray |           |          | 
 stavalues5  | anyarray |           |          | 
Indexes:
    "pg_statistic_relid_att_inh_index" UNIQUE, btree (starelid, staattnum, stainherit)

其中:
starelid:数据列所属的关系或索引Oid
staattnum:数据列编号
stainherit:是否继承表(分区表)
stanullfrac:NULL值所占的比例
stawidth:平均行大小,以字节为单位
stadistinct:非NULL值的唯一值信息.>0,表示多少个唯一值;=0,表示未知;<0,根据重复值出现次数求得:stadistinct = (-1)x(1-stanullfrac)/times(重复次数)
stakindN:在N号slot槽中存储的是哪种类型的统计信息
staopN:在N号槽中所使用的操作符,如"=","<",">"等
stanumbersN:在N号槽中存储的数值类型统计信息
stavaluesN*:在N号槽中存储的数据值(使用anyarray类型)

统计信息类型,在src/include/catalog/pg_statistic.h文件中有相关解释,定义如下:

#define STATISTIC_KIND_MCV  1
#define STATISTIC_KIND_HISTOGRAM  2
#define STATISTIC_KIND_CORRELATION  3
#define STATISTIC_KIND_MCELEM  4
#define STATISTIC_KIND_DECHIST  5
#define STATISTIC_KIND_RANGE_LENGTH_HISTOGRAM  6
#define STATISTIC_KIND_BOUNDS_HISTOGRAM  7

MCV=most common values,最常见值,即高频值
HISTOGRAM=HISTOGRAM,数据分布直方图
CORRELATION=CORRELATION,相关系数,该列未排序的数据分布与排序后的数据分布的相关性
MCELEM=most common elements,与MCV类似,存储的是最常见非NULL值
DECHIST=distinct elements count histogram,描述数组类型列的每行中不同值的分布。
RANGE_LENGTH_HISTOGRAM=length histogram,以行为单位的距离类型列的长度分布。
BOUNDS_HISTOGRAM=与HISTOGRAM类似,面向range-type column

仍以t_grxx表为例说明该表的统计信息
表结构:

testdb=# \d t_grxx
                      Table "public.t_grxx"
 Column |         Type          | Collation | Nullable | Default 
--------+-----------------------+-----------+----------+---------
 dwbh   | character varying(10) |           |          | 
 grbh   | character varying(10) |           |          | 
 xm     | character varying(20) |           |          | 
 xb     | character varying(10) |           |          | 
 nl     | integer               |           |          | 
Indexes:
    "idx_t_grxx_dwbh" btree (dwbh)
    "idx_t_grxx_grbh" btree (grbh)

数据列信息:

testdb=# select attrelid,attname,attnum from pg_attribute where attrelid = 16742 order by attnum;
 attrelid | attname  | attnum 
----------+----------+--------
    16742 | tableoid |     -7
    16742 | cmax     |     -6
    16742 | xmax     |     -5
    16742 | cmin     |     -4
    16742 | xmin     |     -3
    16742 | ctid     |     -1
    16742 | dwbh     |      1
    16742 | grbh     |      2
    16742 | xm       |      3
    16742 | xb       |      4
    16742 | nl       |      5
(11 rows)

dwbh/grbh/xm/xb/nl这5列的编号分别是1/2/3/4/5.
查询常规的统计信息

testdb=# select starelid,staattnum,stainherit,stanullfrac,stawidth,stadistinct from pg_statistic where starelid = 16742;
 starelid | staattnum | stainherit | stanullfrac | stawidth | stadistinct 
----------+-----------+------------+-------------+----------+-------------
    16742 |         1 | f          |           0 |        4 |        9984
    16742 |         2 | f          |           0 |        5 |          -1
    16742 |         3 | f          |           0 |        7 |          -1
    16742 |         4 | f          |           0 |        4 |           2
    16742 |         5 | f          |           0 |        4 |         100
(5 rows)

stanullfrac为0表示没有NULL值
stawidth表示各列的平均行大小,比如dwbh平均行大小为4个字节
stadistinct表示唯一值信息,比如dwbh,10000行中有9984个唯一值,而xb只有2个,grbh和xm则不存在重复值.

查询其他统计信息

testdb=# select starelid,staattnum,stakind1,stakind2,stakind3,stakind4,stakind5 from pg_statistic where starelid = 16742 order by staattnum;
 starelid | staattnum | stakind1 | stakind2 | stakind3 | stakind4 | stakind5 
----------+-----------+----------+----------+----------+----------+----------
    16742 |         1 |        1 |        2 |        3 |        0 |        0
    16742 |         2 |        2 |        3 |        0 |        0 |        0
    16742 |         3 |        2 |        3 |        0 |        0 |        0
    16742 |         4 |        1 |        3 |        0 |        0 |        0
    16742 |         5 |        1 |        3 |        0 |        0 |        0
(5 rows)

第1列(dwbh),在第1/2/3个槽中分别存储了类型为第1/2/3种类型的统计信息,即STATISTIC_KIND_MCV/STATISTIC_KIND_HISTOGRAM/STATISTIC_KIND_CORRELATION,其他槽没有内容
第2列(grbh),在第1/2个槽中分别存储了类型为第2/3种类型的统计信息,即STATISTIC_KIND_HISTOGRAM/STATISTIC_KIND_CORRELATION,其他槽没有内容
第5列(nl),在第1/2个槽中分别存储了类型为第1/3种类型的统计信息,即STATISTIC_KIND_MCV/STATISTIC_KIND_CORRELATION,其他槽没有内容
第一列统计信息

testdb=# \x
Expanded display is on.
testdb=# select staattnum,stakind1,staop1,stanumbers1,stavalues1,
testdb-#                  stakind2,staop2,stanumbers2,stavalues2,
testdb-#                  stakind3,staop3,stanumbers3,stavalues3
testdb-# from pg_statistic 
testdb-# where starelid = 16742 
testdb-#       and staattnum = 1;
-[ RECORD 1 ]---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
staattnum   | 1         -->dwbh
stakind1    | 1         -->STATISTIC_KIND_MCV,高频值统计信息
staop1      | 98        -->"=",查询pg_operator,select * from pg_operator where oid=98;
stanumbers1 | {0.0003}  -->高频值比例,即3/10000
stavalues1  | {24}      -->表示'24'这个值
stakind2    | 2         -->STATISTIC_KIND_HISTOGRAM,直方图
staop2      | 664       -->"<",左闭右开的区间
stanumbers2 |           -->无
stavalues2  | {0,1084,1172,1262,1356,1441,1533,1622,1703,1790,1885,198,207,2165,2259,2351,2447,2530,262,2708,2799,2895,2985,3087,3172,3262,3359,3459,3541,3629,3716,3805,3900,3995,4078,417,4257,4345,4432,4515,461,47,479,489,4985,5069,5154,5244,533,542,5510,5596,5686,5786,587,5963,6053,6145,6232,6323,6412,650,6599,6686,6779,6868,6957,7038,7127,7218,7305,7403,7495,7588,7679,7767,7857,7942,8037,8125,8209,8307,8392,8481,8575,8664,8755,8844,8935,9022,9115,9202,9296,9378,9464,9561,965,9731,982,9906,9999} 
                        -->剔除高频值后的数据分布,假定平均分布,0为MIN值,9999为MAX值
                        -->[0,1084),[1084,1172),...[9906,9999)
stakind3    | 3         -->STATISTIC_KIND_CORRELATION,相关系数
staop3      | 664       -->"<"
stanumbers3 | {0.817163}-->未排序和已排序的数据分布,有81.7%的相关性,值越高,顺序扫描结果越近似于排序
stavalues3  |           -->无

第二列统计信息

testdb=# \x
Expanded display is on.
testdb=# select staattnum,stakind1,staop1,stanumbers1,stavalues1,
                 stakind2,staop2,stanumbers2,stavalues2,
                 stakind3,staop3,stanumbers3,stavalues3
from pg_statistic 
where starelid = 16742 
      and staattnum = 2;
-[ RECORD 1 ]---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
staattnum   | 2         -->grbh
stakind1    | 2         -->STATISTIC_KIND_HISTOGRAM,直方图
staop1      | 664       -->"<"
stanumbers1 |           -->NULL
stavalues1  | {1,1088,11753,12659,1358,14448,15354,16253,17071,17939,18872,19821,20730,21676,22630,23540,24479,25302,26198,27069,27974,2894,29836,30851,31725,32619,33587,34584,35427,36280,37164,3806,39000,39942,40786,41695,42571,43452,44330,4516,46102,46997,47900,48899,49852,50697,5157,52452,533,54204,5512,55963,5686,57860,5871,59633,60544,61468,62331,63252,64134,65016,65993,66870,67798,68687,69568,70391,71285,72205,73066,74045,74962,75881,7679,77675,78573,79434,8038,81276,82125,83078,83939,84822,85764,86654,87557,88450,89352,90249,91171,92030,92979,93797,94665,95628,96510,97328,98212,99074,99998}
                        -->参照第一列
stakind2    | 3         -->STATISTIC_KIND_CORRELATION,相关系数
staop2      | 664       -->"<"
stanumbers2 | {0.816172}-->未排序和已排序的数据分布
stavalues2  |           -->NULL
stakind3    | 0         -->第3个槽无统计信息
staop3      | 0
stanumbers3 | 
stavalues3  |

第五列统计信息

testdb=# select staattnum,stakind1,staop1,stanumbers1,stavalues1,
testdb-#                  stakind2,staop2,stanumbers2,stavalues2,
testdb-#                  stakind3,staop3,stanumbers3,stavalues3
testdb-# from pg_statistic 
testdb-# where starelid = 16742 
testdb-#       and staattnum = 5;
-[ RECORD 1 ]-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
staattnum   | 5             -->nl
stakind1    | 1             -->STATISTIC_KIND_MCV,高频值统计信息
staop1      | 96            -->"="
stanumbers1 | {0.0117667,0.0117333,0.0113,0.0112667,0.0112333,0.0110333,0.0109333,0.0109333,0.0109333,0.0108333,0.0108,0.0108,0.0107667,0.0107667,0.0107333,0.0107333,0.0107333,0.0107,0.0106333,0.0106333,0.0106333,0.0105667,0.0105333,0.0105333,0.0105,0.0104667,0.0104667,0.0104333,0.0104333,0.0103667,0.0103,0.0102667,0.0102333,0.0102333,0.0102333,0.0102333,0.0102,0.0102,0.0102,0.0101667,0.0101667,0.0101333,0.0101,0.0101,0.0100667,0.0100333,0.0100333,0.01,0.00996667,0.00996667,0.00996667,0.00996667,0.00993333,0.00993333,0.00993333,0.00993333,0.0099,0.00986667,0.00983333,0.00983333,0.0098,0.0098,0.0098,0.00973333,0.0097,0.00966667,0.00966667,0.00963333,0.00963333,0.00963333,0.00963333,0.0096,0.0096,0.00956667,0.00953333,0.0095,0.0095,0.0095,0.0095,0.00943333,0.0094,0.00936667,0.00936667,0.00933333,0.00933333,0.0093,0.00916667,0.00916667,0.00916667,0.00913333,0.0091,0.0091,0.0091,0.00906667,0.009,0.00896667,0.00893333,0.00876667,0.00876667,0.0079}
                            -->每个高频值的出现频率
stavalues1  | {73,40,7,51,15,8,14,74,100,53,77,89,33,42,26,29,78,11,16,38,58,18,27,95,49,34,92,65,87,30,32,28,23,62,82,96,1,50,60,36,70,52,9,37,84,54,61,5,59,72,80,93,35,66,79,81,55,68,88,98,13,47,56,22,24,39,43,3,21,75,85,25,44,71,86,4,20,83,97,91,99,63,94,17,46,57,48,67,69,90,2,6,45,19,10,64,76,31,41,12}
                            -->高频值
stakind2    | 3             -->STATISTIC_KIND_CORRELATION,相关系数
staop2      | 97            -->"<",整型比较,664是字符型比较
stanumbers2 | {0.00562935}  -->0.5%的相关性,相关性较低
stavalues2  |               -->NULL
stakind3    | 0             -->无相关信息
staop3      | 0
stanumbers3 | 
stavalues3  |

三、pg_statistic_ext

pg_statistic_ext数据字典表用于存储多列统计信息,需使用CREATE STATISTICS命令创建统计信息,在执行ANALYZE命令时统计.
仍以t_grxx为例,统计dwbh和grbh这两列的信息

testdb=# create statistics sta_t_grxx_dwbh_grbh on dwbh,grbh from t_grxx;
CREATE STATISTICS
testdb=# analyze t_grxx(dwbh,grbh);
ANALYZE

查询多列统计信息

testdb=# \x
Expanded display is on.
testdb=# select * from pg_statistic_ext;
-[ RECORD 1 ]---+-----------------------------------------
stxrelid        | 16742                 -->数据表Oid
stxname         | sta_t_grxx_dwbh_grbh  -->统计信息名称
stxnamespace    | 2200                  -->表空间
stxowner        | 10                    -->统计信息的Owner
stxkeys         | 1 2                   -->列编号,1 2表示dwbh grbh
stxkind         | {d,f}                 -->STATS_EXT_NDISTINCT/STATS_EXT_DEPENDENCIES
stxndistinct    | {"1, 2": 100000}      -->STATS_EXT_NDISTINCT统计信息
stxdependencies | {"1 => 2": 0.040900, "2 => 1": 1.000000} -->STATS_EXT_DEPENDENCIES统计信息

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