Create pipestat iter for dbbackend

[donaldcampbelljr]

may be useful in tandem with seq col project

[donaldcampbelljr]

given filter conditions, returned paged iterator.
pipestat currently has a dynamic_filter function which modifies the sql statement before execution.

[nsheff]

Here's an implementation of this I wrote for seqcolapi.

https://github.com/refgenie/seqcolapi/blob/91326936f2dcd847ba8be28e703a5c0d77377429/seqcolapi/scconf.py#L24-L68

It may just work to take these functions and stick them on the class in pipestat. Then, it will be an iterable MutableMapping with caching.

One thing we should change, though: this code is based on an offset-based pagination. I think we should switch to a cursor-based pagination, which will be more performant -- and in fact, since @donaldcampbelljr already implemented this in pipestat, we should just use that for the 'next page' functionality here.

[donaldcampbelljr]

I modified pipestat's iterator to use the cursor position and limit.

It can be used like so:

psm_iterable = psm.__iter__(limit=100, cursor=5)

for i in psm_iterable:
   print(i) # prints the record

a = next(psm_iterable)

Because pipestat inherits from mutable_mapping, I could only modify __iter__ and not __next__.