VARCHAR / VARGRAPHIC 列的分佈統計資訊？

似乎只有前 n 個字元用作 varchar/vargraphic 列的 colvalue。從我下面的測試來看，它似乎是 33/16。長度是否記錄在某處？我試圖搜尋文件，但找不到任何東西（可能是搜尋錯誤的東西）

簡單的測試案例：

DROP TABLE LEJO0004.T;
CREATE TABLE LEJO0004.T (
       ID BIGINT NOT NULL GENERATED ALWAYS AS IDENTITY 
           ( START WITH 1 INCREMENT BY 1 MINVALUE 1 MAXVALUE 9223372036854775807 NO CYCLE CACHE 20 NO ORDER ),
       TCOL_C VARCHAR(64) NOT NULL,
       TCOL_G VARGRAPHIC(64) NOT NULL
);

insert into LEJO0004.T (tcol_c, tcol_g)
with n100 (n) as ( values 0 union all select n+1 from n100 where n<100 )
  , n10  (n) as ( values 0 union all select n+1 from n10 where n<10 )
  , n1   (n) as ( values 0 )
select 'commons.domain.regel.avbrottsregel'
    , 'commons.domain.regel.avbrottsregel'
from n100
union all
select 'commons.domain.regel.grupp.overfors.nya.endast.som.merit'
    , 'commons.domain.regel.grupp.overfors.nya.endast.som.merit'
from n10
union all
select 'commons.domain.regel.ingar.i.grupp.overfors.till.nya'
   ,  'commons.domain.regel.ingar.i.grupp.overfors.till.nya'
from n1;

runstats on table lejo0004.T with distribution on all columns;

select cast(colname as varchar(15))
   ,  type
   ,  seqno
   ,  cast(colvalue as varchar(40))
   ,  valcount
from sysstat.COLDIST
where tabschema = 'LEJO0004'
 and tabname = 'T'
 and colname in ('TCOL_C', 'TCOL_G')
 and colvalue is not null
 and type = 'F'
order by type, colname, seqno;

結果是：

TCOL_C  F 1 'commons.domain.regel.avbrottsrege' 101
TCOL_C  F 2 'commons.domain.regel.grupp.overfo'  11
TCOL_G  F 1 g'commons.domain.r'                 113

Q似乎是一樣的

values char_length(g'commons.domain.r' using codeunits32)
16

values char_length('commons.domain.regel.avbrottsrege' using codeunits32)
33

測試：

db2level
DB21085I  This instance or install (instance name, where applicable: 
"lejo0004") uses "64" bits and DB2 code release "SQL11050" with level 
identifier "0601010F".
Informational tokens are "DB2 v11.5.0.0", "s1906101300", "DYN1906101300AMD64", 
and Fix Pack "0".
Product is installed at "/home/lejo0004/sqllib".

它是“部分記錄”/“沒有真正記錄”的 AFAIK。更具體地說，有 APAR：

IT13369：在公共前綴大於 32 字節的字元串列上收集分佈統計資訊時的次優查詢性能

提到它。我假設您真正關心的是查詢的基數估計，該查詢在具有公共前綴的列上具有謂詞，在這種情況下，您可以嘗試使用它的解決方法（即不收集該列的分佈統計資訊）

引用自：https://dba.stackexchange.com/questions/254793