Performance of is_null using different representations

[Zhichao Liang]

Hi,

  For the performace of is_null using different representations, I can see a comment in bit_pointer.h says that using simple booleans gets better

performance results than bit representation. I just wonder how much better of using simple booleans instead of bit representation. Any accurate

test result can prove that  claim? Considering the space efficiency, I'd like to use bit representation if it doesn't perform too bad.

Best Regards!

Zhichao

[Piotr Tabor]

Hi Zhichao, 

When we were doing this experiments, we were optimizing performance of functions like IF(IS_NULL(x), y, z). 

The problem here is that regular bool columns in views are not represented as bit-encoding (and this was found as a lot of effort requiring task). 

In this context IS_NULL call required conversion from bits to bytes (expansion) that seems to introduce slowness (we didn't implement that with SSE at that time). 

Piotr

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[Milosz Tanski]

Zhicaho,

We're not using any of the SuperSonic serialization facilities instead
we have our own DiskScan and IndexScan operations that use our own
serialization (columns stored in compressed extents, ala many
filesystems). However, for bools (and nulls) we store store as an
"index" ... eg, we're using the compressed bitmap encoding that we use
in indexing. If want to use bit-vectors you might want to start with
similar technique.

Best,
- Milosz