Eager and Lazy Aggregation

Aggregate processing is one of the most useful operations in DBMS environments. It summarizes these values:

• The minimum, maximum, sum, or average value of a column in a specified set of rows

• The count of rows that match a condition

• Other statistical functions

In SQL, aggregate processing is performed using the aggregation functions min(), max(), count(), sum(), and avg(), and group by and having clauses. The SQL language implements two aggregate processing types, vector aggregation and scalar aggregation. A select-project-join (SPJ) query illustrates these two types of aggregate processing:

select r1, s1
from r, s
where r2 = s2