On Wed, 02 Jul 2008 15:24:38 +0800 Craig Ringer <craig@xxxxxxxxxxxxxxxxxxxxx> wrote: > Tom Lane wrote: > > =?UTF-8?Q?Bj=C3=B8rn?= T Johansen <btj@xxxxxxxxxx> writes: > >> What does this mean and how can it be fixed? > > > > Reduce the number of columns in your SELECTs? > > > > This whiffs to me of excruciatingly bad schema design. How could you > > possibly need upwards of a thousand columns in a query result? > > IMHO reasonable column counts are O(10), not O(bignum). > > (I'm pretty new to Hibernate, so I can only share my general > understanding, but:) > > One possible reason is that sometimes tools like Hibernate like to fetch > records from multiple related tables in the database in one query with > chained left joins. They then scan the results and eliminate duplicates > where appropriate. > > It sounds horrifying, but it can actually be very fast where fairly > small data sets are being fetched from highly normalized tables with > appropriate indexes. In other circumstances, however, like when there > are very high row counts or lots of fields being returned, it's a very > bad strategy. > > My guess is that they haven't told Hibernate to use an appropriate > fetching strategy (multiple SELECTs) for the data they're trying to > load, and for some reason Hibernate is choosing a left join fetch. If > they apply the appropriate annotations to their Hibernate data model or > adjust their HQL queries to avoid "left join fetch" they might find that > the problem goes away - and performance improves significantly. > > -- > Craig Ringer > Ok, guess we have to look at our Hibernate config.... (we are only using default fetching strategy...) Thx... BTJ
