Quote from documentation
The LOB datatypes for character data are CLOB and NCLOB. They can store up
to 8 terabytes of character data (CLOB) or national character set data (NCLOB).
and this is another quote from same page:
The CLOB and NCLOB datatypes store up to 128 terabytes of character data in the database. CLOBs store database character set data, and NCLOBs store Unicode national character set data.`
I am little confused, there is some misunderstanding in documentation or i miss something?
The difference stems from the fact that you can define LOBs with different "chunk" sizes. Plus their maximum size is limited by the number of database blocks used for them. If you create a database (or tablespace) with a larger blocksize this means a LOB can contain more data.
From the manual:
CLOB objects can store up to (4 gigabytes -1) * (the value of the CHUNK parameter of LOB storage) of character data
And the next sentence describes the relation to the blocksize:
If the tablespaces in your database are of standard block size, and if you have used the default value of the CHUNK parameter of LOB storage when creating a LOB column, then this is equivalent to (4 gigabytes - 1) * (database block size).
Related
Why does the database size reduce in PostgreSQL post migration from Oracle schema having lob, clob and blob datatypes
The main reason is that Postgres by default compresses values that are bigger than (approximately) 2000 bytes of data variable length data types - these are mainly text, varchar and bytea types.
Oracle will only compress the content of LOB columns if you are using the Enterprise Edition and enable the compression when defining the LOB column (the most important part is to use SecureFile instead of BasicFile).
Most probably your LOB columns where defined without using compression in Oracle and contain many values bigger than 2000 bytes, that's why you see a reduction in size due to Postgres' automatic compression.
Setup
I have an oracle table that has couple of attributes and a CLOB datatype. The table below I have create with two ways ,each of which should give the same behavior.
CREATE TABLE DEMO(
a number (10, 2),
data CLOB
)
CREATE TABLE DEMO(
a number (10, 2),
data CLOB
) LOB (data) Stored AS (STORAGE IN ROW ENABLED)
Scenario
As per the oracle documentation when the CLOB is greater the 4000 bytes it will be stored outline else inline.
When I store the data in this table for a clob value say "Hello" and then I see the segment information for the "Demo table" and "Demo table LOB segment" , it shows that all the data is going to table and no new blocks are being consumed in the Lob Segment.
When I store a bigger data with total character less than 1500 , then also I get the same behavior as above.
But when I store a data with total character > 2000 and < 3000 , then the LOB data is going to the LOB segment even though total character are less than 3000.
Question
Why is the data smaller than 3000 characters is going to the LOB Segment ? . Is that each character takes 2 bytes , which justifies that data till 1500 is going to the data instead of Log Segment.
Problem
Lots of disk space is getting wasted because of the LOB Table , since the CHUNK size is 8kb and the data per block will always be around 3 - 4K character and in some cases exceeding that. So essential for each row 4Kb space is wasted and in out case of 20 mn rows , its running in 50's of GBs
This may explain the above behaviour..
"The CLOB and NCLOB datatypes store up to 4 gigabytes of character data in the database. CLOBs store database character set data and NCLOBs store Unicode national character set data. For varying-width database character sets, the CLOB value is stored in the database using the two-byte Unicode character set, which has a fixed width. Oracle translates the stored Unicode value to the character set requested on the client or on the server, which can be fixed-width or varying width. When you insert data into a CLOB column using a varying-width character set, Oracle converts the data into Unicode before storing it in the database."
http://docs.oracle.com/cd/B10500_01/server.920/a96524/c13datyp.htm#3234
Does anybody know how SecureFile stores LOBs into chunks? The documentation says the CHUNK parameter is only advisory (http://docs.oracle.com/cd/E11882_01/appdev.112/e18294/adlob_smart.htm#CIHIDGJA).
I did some initial tests, it looks like Oracle uses minimally one block per LOB (EDIT: if it is stored in the lobsegment). Is that right?
CREATE TABLE sftest (mylob CLOB)
LOB (mylob) STORE AS SECUREFILE sftest_mylob (DISABLE STORAGE IN ROW);
INSERT INTO sftest SELECT object_name FROM all_objects;
11,825 rows inserted
SELECT blocks FROM user_segments WHERE segment_name='SFTEST_MYLOB';
14336
Actually, minimum one block per LOB is incorrect. It can be much smaller, depending on the size of your LOB. From the Oracle docs
LOB values are stored inline when any of the following conditions
apply:
When the size of the LOB stored in the given row is small, approximately 4000 bytes or less, and you either explicitly specify
ENABLE STORAGE IN ROW or the LOB storage clause when you create the
table, or when you do not specify this parameter (which is the
default).
When the LOB value is NULL (regardless of the LOB storage properties
for the column).
Using the default LOB storage properties (inline storage) can allow
for better database performance; it avoids the overhead of creating
and managing out-of-line storage for smaller LOB values. If LOB values
stored in your database are frequently small in size, then using
inline storage is recommended.
Yes, the minimal number of blocks of a SecureFile LOB is 1 (if it is not stored in row).
We are using SecureFile LOBs for a number of years in production, and I found out how to inspect the block number of an individual LOB. First, you need to get the number of extents the LOB has by calling DBMS_LOBUTIL.GETINODE(mylob).EXTENTS, then you can find out for each extent, how many blocks are stored it it with DBMS_LOBUTIL.GETLOBMAP(mylob, myextent).NBLKS, e.g.
SELECT DBMS_LOBUTIL.GETINODE(mylob).LENGTH AS len,
DBMS_LOBUTIL.GETINODE(mylob).EXTENTS AS extents,
DBMS_LOBUTIL.GETLOBMAP(mylob, 0).NBLKS AS nblks
FROM sftest;
LEN EXTENTS NBLKS
34 1 1
24 1 1
42 1 1
We have a report (simple text) to store in Oracle , average case will be less than 4K but some cases exceeding that. So an option is to use CLOB.
It is for logging purposes only, not used in query or update. Only insert once and retireve few times.
Space and overall schema (other tables) performance is of main concern.
I read about CLOB storage allocation format.
We are contemplating using 2 columns, msgV varchar2(4000) and msgC CLOB. When text exceeds 4k then we store into CLOB otherwise usual varchar2 and CLOB remains NULL.
So my question is,
Is this scheme better w.r.t to above performance consideration or simply use CLOB ?
(apart from it entailing more coding work maintaining this condition everywhere)
And what is the space consumed by NULL and Empty CLOBs (or any datatype) ?
Use a clob. If the data in the clob is under 4000 bytes, it will actually be stored inline. See section in link below about LOB's compared to LONG.
Oracle Lob
I create a program to insert Large file into database (around 10M). I choosed BLOB type for objects column in my table.
Now I read BLOB just support binary object with maximoum lengh of 4M.
would you advice me what can I do in this case for upload those object more than 4M?
I am useing Oracle 9i or 10g.
You read something that appears to be incorrect.
Per the Oracle 10g Release 2 documentation:
The BLOB datatype stores unstructured binary large objects. BLOB objects
can be thought of as bitstreams with no character set semantics. BLOB
objects can store binary data up to (4 gigabytes -1) * (the value of the
CHUNK parameter of LOB storage).
If the tablespaces in your database are of standard block size, and if you
have used the default value of the CHUNK parameter of LOB storage when
creating a LOB column, then this is equivalent to (4 gigabytes - 1) *
(database block size).
The maximum size of a LOB supported by
the database is equal to the value of
the db_block_size initialization
parameter times the value 4294967295.
This allows for a maximum LOB size
ranging from 8 terabytes to 128
terabytes.
http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14258/d_lob.htm#i1016062
According to this site,
http://ss64.com/ora/syntax-datatypes.html
BLOB has a max size of 4GB since Oracle 8, so 10MB should be no problem.