PPT Slide

• Path expressions are logically equivalent to subqueries

References and Path Expressions (cont.)

select E.name, E.dept->name, E.dept->mgr->name

from emp E

where E.dept->headcount > 10;

• Actual approach: shared subquery generation (QGM)
  • Compute common paths (prefixes) once to save work
  • Not every SQL context accepts an actual subquery
  • Also need to handle non-serializable locking levels

• Efficiency obtained through query rewrite, e.g.:
  • Subquery to outer-join transformation
  • Outer-join to join transformation where possible

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