В сообщении от Thursday 19 June 2008 17:40:14 Dennis Cote написал(а): > Stephen Woodbridge wrote: > > Well if the index requires fewer page reads then it should be > > proportionally faster. For example if you can only get 5 rows on a page > > but 25 index entries, you have 1/5 the number of pages to read. > > Yes, that is true, but this effect is offset by the fact that these > index pages compete with the table pages for the finite space in the > page cache. The speed of counting is usually only an issue with large > databases where the table doesn't fit in the cache. When loading index > pages to speed the count() you are also ejecting table pages that might > be used for the next data query, and hence slowing that data query down > because it now has to reload the data pages. > > Adding such an index also slows down all insert, update, and delete > operations because they must modify the index as well as the table.
I'm testing database size 100 Gb+ on server with 2GB RAM. I need use big databases. For small databases this optimization is not necessary. _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
