As a storage engine, federated does not have the information as to
exactly which columns have been selected. This information is simply
not passed down to the engine.
However, if there is index support for a query then MySQL will indicate
to the engine if it is only interested in the columns of the index, or
in all columns. In this case the engine can optimize the retrieval by
only returning the rows of the index (in most engines this saves
accessing the data record).
So I am not sure how federated works, but you could try creating an
index on the e01_system.category (Id,name). Then you may need to force
the use of the index by modifying your query, for example:
select Id,name from e01_system.category where id > 0
On Aug 9, 2006, at 8:26 AM, wangxu wrote:
who can help me?
----- Original Message -----
From: "wangxu" <[EMAIL PROTECTED]>
To: <mysql@lists.mysql.com>
Sent: Wednesday, August 02, 2006 5:29 PM
Subject: question about the query against to the federated engine
tables
I have a question about the query against to the federated engine
tables. As the following example:
"select Id,name from e01_system.category"
Note: the "e01_system.category" is a federated table.
I select only tow fields from the federated table "category" with
the above sql statement. But it seemed as if gets all the fields of
the table (the table has more then ten fields actually), so the
performance of the sql stetement is very slow when the table's size
is very big.I conclude this through monitoring the throughput of the
network with a network tool named "sniffer". I think the federated
table works not as what i expected.
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