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

This is the key information (how to read LOB without extra roundtrips), which is not available in Oracle's documentation I think:

Another option is to bind the LOB on client side as if it was a big RAW/VARCHAR2. This only works if a maximum size of the LOB can be defined (since the maximum size must be provided at bind time). This trick avoids the extra rountrips: the LOBs are just processed like RAW or VARCHAR2. We use it a lot in our LOB intensive applications.

I had problem with loading simple table (few GB) with one blob column ( 14KB => thousands of rows) and I was investigating it for a long time, tried a lot of lob storage tunings (DB_BLOCK_SIZE for new tablespace, lob storage specification - CHUNK ), sqlnet.ora settings, client prefetching attributes, but this (treat BLOB as LONG RAW with OCCI ResultSet->setBufferData on client side) was the most important thing (persuade oracle to send blob column immediately without sending lob locator at first and loading each lob separately based on lob locator.

Now I can get even ~ 500Mb/s throughput (with columns < 3964B). Our 14KB blob will be separated into multiple columns - so it'll be stored in row to get almost sequential reads from HDD. With one 14KB blob (one column) I get ~150Mbit/s because of non-sequential reads (iostat: low amount of merged read requests).

NOTE: don't forget to set also lob prefetch size/length:

err = OCIAttrSet(session, (ub4) OCI_HTYPE_SESSION, (void *) &default_lobprefetch_size, 0, (ub4) OCI_ATTR_DEFAULT_LOBPREFETCH_SIZE, errhp);

But I don't know how is it possible to achieve the same fetching throughput with ODBC connector. I was trying it without any success.