Leon Rosenberg wrote:
On Fri, Jan 23, 2009 at 11:45 AM, Chris Wareham
<cware...@visitlondon.com> wrote:
By it's very definition (see Codd or Date), an RDBMS should be capable
of performing joins with good performance. MySQL often struggles to do
so thanks to the poor optimiser, so you had to implement what should be
core functionality of an RDBMS in your application layer.
Sorry, but by the very definition JOINs are slow, and no database in
the world will ever be able to make them fast :-)
Avoiding JOINs is often a good architectural advice (taking in account
a previous post of the contrary).
For example if you need all orders by user with name Chris, you will
ALWAYS be faster if you first retrieve the userid, and than the orders
of the userid.
So you perform two queries from the application layer? You are basically
doing a join by hand - the cost of those two round trips to the database
will lose to a single query with a join, unless you've not setup
adequate indexes and your tables have a huge number of rows in them.
No query optimizer in the world can perform better than the develop,
simply because it lacks the knowledge a good developer should have
about the semantic of his application.
Yes, and occasionally I can produce "better" assembler code than my C
compiler. However, in your case you're ignoring the fact that any RDBMS
worth its salt is going to have cached a lot of data in memory, in a way
that is likely to be as fast to access if not faster than the same data
cached and accessed at the application layer.
In the system I currently work on, there was a gem of a code comment by
a previous developer that said something like "perform these queries
separately as the database cannot optimise them". Normalising the tables
meant the database could. If your joins are costing you so much
performance, then I'd humbly suggest your database design is screwed.
my 2 cent as an addition to Jonathan's 2, make it 4 against the joins :-)
Sadly your four cents aren't even legal tender here ;-)
Chris
--
Chris Wareham
Senior Software Engineer
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