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Don Burleson Blog 







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 lengthb(to_char(substr(,1,4000)))

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

   nvl((sum(dbms_lob.getlength("CLOB_TABLE"))),0) as bytes

   nvl((sum(dbms_lob.getlength("CLOB_TABLE"))),0) as bytes

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:


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:  

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




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