Re: POC, WIP: OR-clause support for indexes

On 28/3/2024 16:54, Alexander Korotkov wrote:
> The current patch has a boolean guc enable_or_transformation.
> However, when we have just a few ORs to be transformated, then we
> should get less performance gain from the transformation and higher
> chances to lose a good bitmap scan plan from that.  When there is a
> huge list of ORs to be transformed, then the performance gain is
> greater and it is less likely we could lose a good bitmap scan plan.
> 
> What do you think about introducing a GUC threshold value: the minimum
> size of list to do OR-to-ANY transformation?
> min_list_or_transformation or something.
I labelled it or_transformation_limit (see in attachment). Feel free to 
rename it.
It's important to note that the limiting GUC doesn't operate 
symmetrically for forward, OR -> SAOP, and backward SAOP -> OR 
operations. In the forward case, it functions as you've proposed. 
However, in the backward case, we only check whether the feature is 
enabled or not. This is due to our existing limitation, 
MAX_SAOP_ARRAY_SIZE, and the fact that we can't match the length of the 
original OR list with the sizes of the resulting SAOPs. For instance, a 
lengthy OR list with 100 elements can be transformed into 3 SAOPs, each 
with a size of around 30 elements.
One aspect that requires attention is the potential inefficiency of our 
OR -> ANY transformation when we have a number of elements less than 
MAX_SAOP_ARRAY_SIZE. This is because we perform a reverse transformation 
ANY -> OR at the stage of generating bitmap scans. If the BitmapScan 
path dominates, we may have done unnecessary work. Is this an occurrence 
that we should address?
But the concern above may just be a point of improvement later: We can 
add one more strategy to the optimizer: testing each array element as an 
OR clause; we can also provide a BitmapOr path, where SAOP is covered 
with a minimal number of partial indexes (likewise, previous version).

-- 
regards,
Andrei Lepikhov
Postgres Professional