Distribution statistics on VARCHAR / VARGRAPHIC columns?

Question

It appears that only the first n characters is used as colvalue for varchar/vargraphic columns. From my test below it appears to be 33/16. Is the length documented somewhere? I tried to search the documentation, but can't find anything (probably searching for the wrong thing)

Simple test case:

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;

Result is:

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

Appears to be the same for Q

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

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

Tested on:

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".

Answer 1

It is "partially documented"/"not really documented" AFAIK. More specifically, there is APAR:

IT13369: SUB-OPTIMAL QUERY PERFORMANCE WHEN DISTRIBUTION STATS COLLECTED ON STRING COLUMN WITH A COMMON PREFIX LARGER THAN 32 BYTES

that mentions it. I assume what you actually care about are cardinality estimates for a query that has predicate on a column that has a common prefix in which case you might try the workaround from it (i.e. NOT collect distribution stats for that column)