Faster hash joiner with vectorized execution (opens in new tab)

I have a hobby project to write an analytics DB that uses ISPC for vectorized execution. Currently not much (sums are real easy) but I really wonder if it could reduce the effort to vectorize these sorts of things.

It's always good practice to dig into a deep project like this with some napkin estimates of how much you stand to gain, and how much overhead you can afford to spend setting yourself up for the faster computation. (Not to mention how much of your own time is merited!)

Essentially it's turning :

  LOAD
    DISPATCH
    OP1
    DISPATCH
    OP2
    ... (once per operation in the expression)
  STORE
  ... (once per row)

  DISPATCH
    LOAD
    OP1
    STORE
    LOAD
    OP1
    STORE
    ... (once per row)
  DISPATCH
  ... (once per operation in the expression)

If you can generate code it's even better to fuse the operations, to get something like :

  LOAD
    OP1
    OP2
    ...
  STORE
  LOAD
  ...

For example on Haswell you can only issue 1 store per cycle, so if OP is a single add you're leaving up to 3/4 of your theoretical ALU throughput on the table.