Oracle 10g small Blob or Clob not being stored inline?

Question

According to the documents I\'ve read, the default storage for a CLOB or BLOB is inline, which means that if it is less than approx 4k in size then it will be held in the table.

Answer 1

There are two indirections when it comes to CLOBs and BLOBs:

1. The LOB value might be stored in a different database segment than the rest of the row.

2. When you query the row, only the non-LOB fields are contained in the result set and accessing the LOB-fields requries one or more additional round trips between the client and the server (per row!).

I don't quite know how you measure the execution time and I've never used Oracle Monitor, but you might primarily be affected by the second indirection. Depending on the client software you use, it is possible to reduce the round trips. E.g. when you use ODP.NET, the parameter is called InitialLobFetchSize.

Update:

One one to tell which of the two indirections is relevant, you can run your LOB query with 1000 rows twice. If the time drops significantly from the first to the second run, it's indirection 1. On the second run, the caching pays off and access to the separate database segment isn't very relevant anymore. If the time stays about the same, it's the second indirection, namely the round trips between the client and the server, which cannot improve between two runs.

The time of more than 8 seconds for 1000 rows in a very simple query indicate it's indirection 2 because 8 seconds for 1000 rows can't really be explained with disk access unless your data is very scattered and your disk system under heavy load.