

Symbol  Name of operator  Operation 

tablescan(R)  Table scan  Reads blocks of the input relation R in  one block at a time. 
indexscan(R)  Index scan  Reads blocks of the input relation R in using some index on the relation R 
σ_{cond}  Selection  Selects the tuples that satisfies the condition cond 
π_{attrs}  Projection 
Extracts
the attributes
in attrs from
a tuple
Note: result of π is a bag (possible duplicates !!!) 
⋈_{cond}  Theta Join  Joins 2 relations on the condition cond 
⋈  Natural Join  Equijoins 2 relations on the attributes than have the same name 
×  Product  Computes the (cartesian) product of 2 relations 
δ  Duplicate Elmination  Removes the duplicate tuples from a bag of tuples (the result is a set) 
γ_{L}  Grouping  Form gorups of tuples on common attribute values and apply the function L on each group 
∪_{S}  Union  Computes the set union of 2 sets (no duplicates) 
∪_{B}  Union  Computes the bag union of 2 sets (allows duplicates) 
∩_{S}  Union  Computes the set intersection of 2 sets (no duplicates) 
∩_{B}  Union  Computes the bag intersection of 2 sets (allows duplicates) 
−_{S}  Union  Computes the set difference of 2 sets (no duplicates) 
−_{B}  Union  Computes the bag difference of 2 sets (allows duplicates) 
