11. A system for performing query optimization having stored thereon computer readable instructions, said system comprising: a processor; a view statistics storage comprising storage for at least one view statistic, each of said at least one view statistic relating to a corresponding view and collected pursuant to a user request, the user request comprising a completeness level, the completeness level indicating a sampling of data from the view, as selected by the user, that is to be used in collecting the view statistic; a query optimizer, operably connected to said view statistics storage, for developing at least one candidate plan for a query and using at least one of said at least one view statistic to estimate the cost of said candidate plan, wherein said query optimizer estimates the cost of said candidate plan using at least one transformation rule to transform a first expression comprising all or part of said candidate plan and having an associated first expression estimate quality indicator into an equivalent second expression having an associated second expression estimate quality indicator, where said second expression comprises a reference to one or more views matching the first expression, wherein said second expression and said first expression have an equivalent cardinality, and wherein said query optimizer is further adapted to select a recommended execution plan comprising one of the first expression and the second expression having the higher of the first expression estimate quality indicator and the second expression estimate quality indicator.
11. A system for performing query optimization having stored thereon computer readable instructions, said system comprising: a processor; a view statistics storage comprising storage for at least one view statistic, each of said at least one view statistic relating to a corresponding view and collected pursuant to a user request, the user request comprising a completeness level, the completeness level indicating a sampling of data from the view, as selected by the user, that is to be used in collecting the view statistic; a query optimizer, operably connected to said view statistics storage, for developing at least one candidate plan for a query and using at least one of said at least one view statistic to estimate the cost of said candidate plan, wherein said query optimizer estimates the cost of said candidate plan using at least one transformation rule to transform a first expression comprising all or part of said candidate plan and having an associated first expression estimate quality indicator into an equivalent second expression having an associated second expression estimate quality indicator, where said second expression comprises a reference to one or more views matching the first expression, wherein said second expression and said first expression have an equivalent cardinality, and wherein said query optimizer is further adapted to select a recommended execution plan comprising one of the first expression and the second expression having the higher of the first expression estimate quality indicator and the second expression estimate quality indicator. 12. The system of claim 11 , where at least one of said at least one view statistic comprises a cardinality for a corresponding view.