What does it means TableCardinality in execution plan?
I am looking at data base tuning performances
Thanks
What does it means TableCardinality in execution plan? I am looking at data base
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What does it means TableCardinality in execution plan?
I am looking at data base tuning performances
Thanks
Cardinality is the number of UNIQUE values for a given table / column (so it’s not surprising that is equal to the number of entries in the primary key index as that index is most likely clustered). The cardinality of the index or table is usefull to sql server as it allows the query optimiser to make educated guesses about the possible best plans to use when referencing that table.
When the optimiser has to take your sql code and work out what to do with it, it will consider alternative plans before settling on the one that it will use to actually retrieve the data. In most real world cases the number of possible plans is too large for sqlserver to calculate the absolute best plan via sampling all possibles so the optimiser will use statistical analysis to determin a “good enough” plan.
Cardinality is one of the metrics it uses to work out such a plan as it may use this to derermine which physical joins to use (hashmaps or loops for example)