* Opgave 8.3 fra bogen

  #+begin_quote
  Consider the parallel hash join algorithm in Sect. 8.4.1.2.
  Explain what the build phase and probe phase are.
  Is the algorithm symmetric with respect to its input relations?
  #+end_quote

** Explain build and probe phases

   The build phase hashes the =R= phase into $p$ partitions.
   Then it sends the partitions to their respective nodes.
   The nodes in $[1, p]$ each receive the partitions and creates a local hash table for $R_j$.

   In the probe phase all nodes do the same with =S= and
   the nodes $[1, p]$ receives the partitions of =S= joins it with =R=.

** Is the algorithm symmetric?

   No it's not, be inner join(thus =R=) in the build phase, must be completely stored and hashed.
   It can't start doing join on it while it's receiving.

   A symmetric algorithms allows changing the order in which inputs are consumed.

* Opgave 8.7 fra bogen

  #+begin_quote
  Consider a nine way join (ten relations are to be joined),
  calculate the number of possible right-deep, left-deep, and bushy trees,
  assuming that each relation can be joined with anyone else.
  What do you conclude about parallel optimization?
  #+end_quote

  With right-deep we can join them in $10!$ different ways, as we just order them differently down the spine.
  The same for left-deep.

  For bushy trees we look at the leafs of the tree.
  Here we can have $10!$ different orders again.
  But we multiply this with the number of ways to create a 5 leaf binary tree, which i count as 3.

  Therefore the end result is $2 * 10! + 3 * 10! = 5 * 10! = 18144000$.
  Hmm that does not seem right.