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Find the length of a CLOB

Oracle Database Tips by Donald BurlesonMarch 14, 2015

Question:  I need to estimate the average length of a CLOB column in my table.  I Have found many ways to estimate a CLOB length:

- The lengthb function can be used to show the length in bytes of a CLOB column name:

select
   select lengthb(to_char(substr(,1,4000)))
from
   mytab;

- dbms_lob.getlength will also show the average length of a CLOB column:

select
   nvl((sum(dbms_lob.getlength("CLOB_TABLE"))),0) as bytes
from
   clob_table;


select
   nvl((sum(dbms_lob.getlength("CLOB_TABLE"))),0) as bytes
from
   clob_table;

Which one of these methods gives the correct CLOB length?  How can I check the length of a Long Datatype in a table column?

Answer:  As you have demonstrated, there are many ways to get the average length of a CLOB column.  The best-practices approach is to use the dbms_lob.getlength procedure. 

Oracle has many types of large objects (BLOB, CLOB, LONG, LONG RAW), and you need to run a utility to find the actual used space in a long object (CLOB) data type length within a table. 

There are a few ways to compute the actual CLOB data length for a specific row, but I prefer the dbms_lob package:

select
   (dbms_lob.getlength(colname))/1024/1024
from
   mytable;

Here is a working example of computing the row length in a case where the name and datatype of each row are known in advance:  

select
nvl(dbms_lob.getlength(CASE_DATA),0)+1 +
     nvl(vsize(CASE_NUMBER   ),0)+1 +
     nvl(vsize(CASE_DATA_NAME),0)+1 +
     nvl(vsize(LASTMOD_TIME_T),0)+1
     "Total bytes per row"
from
    mytab
where
    case = 123;

 


 

 

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