Where are Extended Data Types stored? (12c)

Oracle has introduced Extended Data Types in 12c. If you still don’t know what they are and how enable them, refer to the awesome oracle-base blog or the official documentation.

Here, I’ll show where they are stored, and when.

Tables created with extended varchar2 (length >4000)

If the table is created with a varchar2 length above 4000, a LOBSEGMENT and a LOBINDEX segments are also created automatically. But actually, the rows inserted may be short enough to be stored inline.

 

As you can see from the previous output, the LOB segments are almost empty after 1000 rows inserted, whereas the table has 24 blocks. Not enough to be a proof. Let’s try to update the rows with a column size of 1000:

 

A-ah! The row size is increasing but the blocks are actually allocated in the table segment, this prove that the rows are stored inline!

Where’s the break point? When the rows will be migrated to the out-of-line storage?

 

Actually, it’s somewhere between 3960 and 3970 bytes, but it may depend on other factors (I havn’t tested it deeply).

Lesson learned: when you design your table with extended types, you should plan how many rows you expect with a size below 4000, because they can make your row access slower and chained rows higher than expected.

 

Tables created with standard varchar2 and altered afterward

Let’s try almost the same excercise, but this time starting with a table created with a small length (lower than 4000) and altered afterward to make use of the extended size.

So far, so good. Let’s try to extend the column:

The LOB segments have not been created. Meaning that the rows are still stored inline. Let’s try then to update all the rows to a size above 4000:

The blocks have grown in the table segment, the rows are still inline!!

This lead to a huge amount of chained rows!! In this example, 2 blocks per row, but it can be as high as 5 blocks for a single column varchar2(32767) with db_block_size=8192.

So we need to rebuild the table, right?

WRONG! Even after a rebuild the extended varchars are stored INLINE if they have been created as standard varchars. Actually you need to recreate the table and migrate with dbms_redefinition.

Migrating to extended datatypes instead of converting your application to use secure files  can be a disaster for the physical layout of your tables, thus for the performance of your application. Be careful!

 

Update:

Many thanks to Sayan Malakshinov (@xtner) and Franck Pachot (@FranckPachot) for the information :-)

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Ludovico

Oracle ACE and Senior Consultant at Trivadis SA
After being DBA Team Leader in Italy for a while, Ludovico works as Senior Database Specialist on the full Oracle stack (Clusterware, RAC, ASM), MySQL (Replication, DRBD, Galera), SQLServer and PostgreSQL.

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