Skip site navigation (1) Skip section navigation (2)

Re: SOLVED ... Re: Getting rid of a cursor from JDBC .... Re: [PERFORM] Re: HELP: How to tame the 8.3.x JDBC driver with a biq guery result set

From: Dave Crooke <dcrooke(at)gmail(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: pgsql-jdbc(at)postgresql(dot)org, pgsql-performance <pgsql-performance(at)postgresql(dot)org>, Scott Carey <scott(at)richrelevance(dot)com>
Subject: Re: SOLVED ... Re: Getting rid of a cursor from JDBC .... Re: [PERFORM] Re: HELP: How to tame the 8.3.x JDBC driver with a biq guery result set
Date: 2010-04-20 19:29:28
Message-ID: x2zca24673e1004201229n653f4e69vd274af4a4aa22d09@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-jdbcpgsql-performance
I don't want to get into a big debate about standards, but I will clarify a
couple of things inline below.

My key point is that the PG JDBC driver resets people's expecations who have
used JDBC with other databases, and that is going to reflect negatively on
Postgres if Postgres is in the minority, standards nothwithstanding, and I
feel badly about that, because PG rocks!

Cheers
Dave

On Tue, Apr 20, 2010 at 11:32 AM, Kevin Grittner <
Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:

> Dave Crooke <dcrooke(at)gmail(dot)com> wrote:
>
> > AFAICT from the Java end, ResultSet.close() is supposed to be
> > final.
>
> For that ResultSet.  That doesn't mean a ResultSet defines a cursor.
> Such methods as setCursorName, setFetchSize, and setFetchDirection
> are associated with a Statement.  Think of the ResultSet as the
> result of a cursor *scan* generated by opening the cursor defined by
> the Statement.
>
> http://java.sun.com/javase/6/docs/api/java/sql/ResultSet.html#close%28%29
>
> Notice that the ResultSet is automatically closed if the Statement
> that generated it is re-executed.  That is very much consistent with
> Statement as the equivalent of a cursor, and not very consistent
> with a ResultSet as the equivalent of a cursor.
>

True, but mechanically there is no other choice - the ResultSet is created
by Statement.executeQuery() and by then it's already in motion .... in the
case of Postgres with default settings, the JVM blows out before that call
returns.

I am not explicitly creating any cursors, all I'm doing is running a query
with a very large ResultSet.


 Again, you're talking about the *results* from *opening* the cursor.
>
> > At a pragmatic level, the PGSQL JDBC driver has a lot of odd
> > behaviours which, while they may or may not be in strict
> > compliance with the letter of the standard, are very different
> > from any other mainstream database that I have accessed from Java
> > .... what I'd consider as normative behaviour, using regular JDBC
> > calls without the need to jump through all these weird hoops, is
> > exhibited by all of the following: Oracle, SQL Server, DB2, MySQL,
> > Apache Derby and JET (MS-Access file-based back end, the .mdb
> > format)
>
> Are you talking about treating the Statement object as representing
> a cursor and the ResultSet representing the results from opening
> the cursor, or are you thinking of something else here?
>

Specific examples:

a. the fact that Statement.executeQuery("select * from huge_table") works
out of the box with every one of those databases, but results in
java.langOutOfMemory with PG without special setup. Again, this is to the
letter of the standard, it's just not very user friendly.

b. The fact that with enterprise grade commercital databases, you can mix
reads and writes on the same Connection, whereas with PG Connection.commit()
kills open cursors.

The fact that I've been using JDBC for 12 years with half a dozen database
products, in blissful ignorance of these fine distinctions in the standard
until I had to deal with them with PG, is kinda what my point is :-)

I understand the reasons for some of these limitations, but by no means all
of them.


> Huh.  I dropped PostgreSQL into an environment with hundreds of
> databases, and the applications pretty much "just worked" for us.
> Of course, we were careful to write to the SQL standard and the JDBC
> API, not to some other product's implementation of them.
>

True, but not everyone can hire every developer to be a JDBC / SQL language
lawyer. All of our SQL is either ANSI or created by the Hibernate PGSQL
adapter, with the exception of a daily "VACUUM ANALYSE" which I added ;-)

I do believe that when there are two ways to implement a standard, the "it
just works" way is far preferable to the "well, I know you probably think
this is a bug, because 90% of the client code out there chokes on it, but
actually we are standards compliant, it's everyone else who is doing it
wrong" way.

I used to work at a storage startup that did exactly the latter, using an
obscure HTTP/1.1 standard feature that absolutely none of the current
browsers or HTTP libraries supports, and so it was a constant source of
frustration for customers and tech support alike. I no longer work there ;-)

It's kinda like making stuff that has to work with Windows - you know
Microsoft doesn't follow it's own standards, but you gotta make our code
work with theirs, so you play ball with their rules.


> (1)  Needing to setFetchSize to avoid materializing the entire
> result set in RAM on the client.
>

I don't understand the rationale for why PG, unlike every other database,
doesn't make this a sensible default, e.g, 10,000 rows ... maybe because the
locks stay in place until you call Connection.close() or Connection.commit()
? ;-)


>
> (2)  Fixing a race condition in our software which was benign in
> other products, but clearly my own bug.
>

Been there and done that with code developed on single-threaded DB's (JET /
Derby) ... not what I'm griping about here though, the base code with no
extra JDBC setup calls works perfectly against Oracle.


> Just out of curiosity, did you discuss that on the PostgreSQL lists?
> Can you reference the thread(s)?
>

No, I was in a hurry, and the "just works" model was available with both
MySQL and Berkeley DB, so I didn't see the point in engaging. I felt the in
house paranoia about the MySQL licensing (our CFO) was not justified, and it
was the devil I knew, I was taking a look at PG which was then foreign to me
as a "genius of the and" alternative.


>
> Sure, but I would consider it a step away from that to follow
> MySQL's interpretation of cursors rather than the standard's.
> YMMV, of course.
>

I wouldn't hold MySQL up to be a particularly good implmentation of
anything, other than speed (MyISAM) and usability (the CLI) .... I find
Oracle's JDBC implmentation to be both user friendly and (largely) standards
compliant.

YMMV too :-)

I hope this can be taken in the amicable spirit of gentlemanly debate in
which it is offered, and in the context that we all want to see PG grow and
continue to succeed.

Cheers
Dave

In response to

Responses

pgsql-performance by date

Next:From: Scott MarloweDate: 2010-04-20 19:33:32
Subject: Re: performance change from 8.3.1 to later releases
Previous:From: Scott MarloweDate: 2010-04-20 19:24:34
Subject: Re: Very high effective_cache_size == worse performance?

pgsql-jdbc by date

Next:From: Nikolas EverettDate: 2010-04-20 19:57:18
Subject: Re: SOLVED ... Re: Getting rid of a cursor from JDBC .... Re: [PERFORM] Re: HELP: How to tame the 8.3.x JDBC driver with a biq guery result set
Previous:From: Kevin GrittnerDate: 2010-04-20 16:32:40
Subject: Re: SOLVED ... Re: Getting rid of a cursor from JDBC .... Re: [PERFORM] Re: HELP: How to tame the 8.3.x JDBC driver with a biq guery result set

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group