Locator Algebra (2)

• Select

  • select[p]: L ? L, where p: PL ? Booleanselect[p](x) = {u | u ? x, p(tail(u))}
  • Example: select[nodename(.) = “book”](x) =select[“book”](x)

• Return

  • Corresponds to projectduplicates tail of locator for preserving it insubsequent d-join (see below)
  • return: PL ? PLreturn(u)=concat(u, tail(u))

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