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Query Evaluation (cont)

DBMSs provide several "flavours" of each RA operation.

For example:

  • several "versions" of selection (σ) are available
  • each version is effective for a particular kind of selection, e.g 

    select * from R where id = 100  -- hashing
select * from S                 -- Btree index
where age > 18 and age < 35
select * from T                 -- MALH file
where a = 1 and b = 'a' and c = 1.4

Similarly, π and ⋈ have versions to match specific query types.