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Re: Re: Proposal to fix Statement.executeBatch()

From: Rene Pijlman <rene(at)lab(dot)applinet(dot)nl>
To: pgsql-jdbc(at)postgresql(dot)org
Cc: Barry Lind <barry(at)xythos(dot)com>
Subject: Re: Re: Proposal to fix Statement.executeBatch()
Date: 2001-09-01 20:38:01
Message-ID: h3h2pt4l36vhhk4iq7d1kmct7giacp39pp@4ax.com (view raw or flat)
Thread:
Lists: pgsql-jdbc
On Mon, 27 Aug 2001 22:57:13 -0700, Barry Lind wrote:
>I do not know what the server does if you have autocommit enabled 
>and you issue multiple statements in one try.

As you know, Peter Eisentraut said on hackers that all
statements in a semicolon-separated query string are processed
as one single transaction. So, if in "S1;S2;S3" S2 fails, both
S1, S2 and S3 are rolled back and in effect they have all
failed.

I think this means we can implement your proposal. I've removed
the paragraph saying that we can't and I've added the following
to http://lab.applinet.nl/postgresql-jdbc/#Batch

-+-+-

The current implementation of Statement.executeBatch() in the
JDBC driver does not provide any performance improvement
compared to processing statements individually. This is because
the driver executes the statements one-by-one when
executeBatch() is called, using one round trip per statement. We
intend to reimplement executeBatch() in the following way.

Statement.executeBatch() will send all statements in a single
semicolon separated query string, with only one round trip to
the backend. This will provide a performance improvement, as
intended by the JDBC specification.

The updateCounts array will be set as described below. Note that
the JDBC spec defines the meaning of the following special
values:

-2 the statement was executed successfully but the number of
affected rows is unknown 
-3 the statement failed to execute successfully 

If all statements succeed, executeBatch() returns an
updateCounts array with a row count for each statement in the
batch, however the value for all but the last statement will be
-2. The value for the last statement will be a proper update
count. 
If a statement fails, executeBatch() throws a
BatchUpdateException containing an updateCounts array with a row
count of -3 for each statement in the batch.

Note that the behaviour will be the same when autocommit is
enabled and when it is disabled. Even with autocommit enabled,
the backend will commit or rollback all statements in the
semicolon-separated query string as a unit.

The new implementation of executeBatch() will cause a change in
behaviour of the driver: the driver will no longer return update
counts for all statements in a batch like it currently does, it
will return -2 ("unknown") for most statements. However, this
behaviour is allowed by the JDBC spec and applications should be
prepared to handle it.

-+-+-

I see two more issues we need to decide on...

1) The JDBC spec requires Statement.executeBatch() to throw a
BatchUpdateException if any of the statements does not return an
update count (e.g. is a SELECT). How can we implement this? Do
we need to parse the statements in the JDBC driver to detect
SELECT's? It is a matter of interpretation, but it seems OK to
me to just ignore this and return -2/-3 for SELECT's as well. In
fact, perhaps we should allow SELECT's for function calls!?

2) The reimplementation may cause the driver to send very long
statements to the backend. I heard something about an 8K limit.
In what version of the backend was this limitation removed? I
guess we should implement the new algorithm conditionally, so
we'll only send multi-statement query strings to a backend that
has no statement length limitation.

Regards,
René Pijlman <rene(at)lab(dot)applinet(dot)nl>

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