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Finding the length of a BLOB

Oracle Database Tips by Donald BurlesonMay 1, 2015 Updated 30 October 2015

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

- Join dba_segments into dba_lobs

- dbms_lob.getlength


select
   table_name,
   column_name,
   segment_name,
   a.bytes
from
   dba_segments a
join
   dba_lobs b
using (owner, segment_name)
where
   b.table_name = 'BLOB_TABLE';

ETM_RAW_XML
IFD_XML
SYS_LOB0007260522C00012$$ 87870668800

ETM_RAW_XML
ETM_XML
SYS_LOB0007260522C00011$$ 125199974400

Beware, there are issues where using dbms_lob.getlength does not give the correct BLOB size.   IMPORTANT!  Make sure that you use double quotes when using dbms_lob.getlength or you might get a false response:


select
   nvl((sum(dbms_lob.getlength("BLOB_TABLE"))),0) as bytes
from
   blob_table;


bytes
----------------
83848300852 ~ 78.0898154266179 GB

select
   nvl((sum(dbms_lob.getlength("BLOB_TABLE"))),0) as bytes
from
   blob_table;


bytes
----------------
61907953222 ~ 57.6562743838876 GB

Which one of these methods gives the correct BLOB 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 BLOB 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 (BLOB) data type within a table. 

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

select
   dbms_lob.getlength(new_rec)
from 
   blob_mytab
order by
   dbms_lob.getlength(new_rec) desc ;

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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