Hi Ying,

> I am just very curious why for read-only queries, they will compete for disk
> I/O.

The reason is that your query requires a sequential scan of the full
XML structure. If you run several requests in parallel, all requests
will try to read the same blocks in a slightly delayed order. As a
result, the pointer, which is reading a block on disk, will be moved
again and again, and the resulting pattern is somewhat random.

> It there anyway we can brutely load the whole index and then do the queries.

Due to the nature of your query, there is now way to benefit from
index structures. Instead, I would suggest to create additional
databases that contain all the information you will be frequently
accessing.

Christian