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- Smallest Parent Computing AB
from smallest of its parents ABC and ABD is more efficient.
- Cache Result Compute AB while ABC is still in memory.
- Amortize Scans Compute ABC, ACD, ABD and BCD in
one scan of ABCD.
- Share sorts Share sorting cost across multiple group-bys
in the sort based algorithms.
- Share partition In hash based algorithm, when hash table is
too large, it must be partitioned. This partitioning cost can be shared
across multiple group bys.
- Overlap optimization 1 An aggregate can be computed from a
sorted ascendent aggregate in sorted order.
- Overlap optimization 2 An existing sort order on an aggregate
can be used while computing others from it. For example, consider
ABD to be computed in the same sorted order from ABCD, which
is sorted in the same order. In that case, one need not resort ABCD
to compute ABD but use the existing sort order of ABCD.
DBMS
1999-04-01