Make sure statistics are available for tables used in queries, especially for the columns used in restrictions or joins between tables. The timestamp of the last generated statistics is available in the iistats database view (select * from iistats).

When the distribution of data values in the tables and columns that the statistics report on has changed significantly from when they were last gathered, you may need to regenerate statistics. The change in distribution might happen if a large data load has taken place and the new data has a different value distribution to the values that were already in the table. This can happen easily with timestamped data, for example, since timestamps for new data will by definition be of a different distribution than that of older data.

Histograms are automatically generated only for those columns that do not already have histograms stored in the catalog.

For more information, see Generating Statistics.