Re: Row pattern recognition

Hi.

Please check the attached refactoring for nodeWindowAgg.c and execRPR.c.
It's based on https://github.com/assam258-5892/postgres/commits/RPR.

What is being changes:
1. ExecRPRFreeContext: we should also reset other field values.
2. nfa_update_absorption_flags() now takes the WindowAggState and walks
the context list itself. This is more intuitive, I think.
I did the similar thing for nfa_absorb_contexts.
3. Remove some duplicated Asserts; preceding callers have already done
the equivalent Asserts.
4. Some local variable, used only once, should be just removed.
5. Add some elog(ERROR) to avoid circular winstate->nfaContext,
RPRNFAContext->states.
6. Only ExecRPRProcessRow uses the variable frameOffset, refactor to
make it as a local variable.
7. ExecRPRStartContext->nfa_context_make. nfa_context_make() used to return a
bare struct, leaving the initial state, matchStartRow, and active-list linkage
for the caller to fill in, that is not intuitive, it looks like a half-built
context. Have nfa_context_make() return a fully-formed one: allocate the
initial state, set the start row, and append it to the list tail.
8. ExecRPRGetHeadContext(pos) external function can be removed.
9. Refactoring update_reduced_frame. Now update_reduced_frame code
flow is more intuitive: look up or
create the context for pos, drive the NFA forward with
advance_reduced_frame_nfa(), then record the match result.
10. Restructure get_reduced_frame_status as a single decision tree: first
"no record" (start < 0), then "the record's own row" (pos == start),
where length alone gives the verdict (-1 unmatched, 0 empty match,
>= 1 frame head), then the range test for everything else.

Summary: The code flow in update_reduced_frame, advance_reduced_frame_nfa, and
get_reduced_frame_status is now much more intuitive. Using elog(ERROR) prevents
us from getting stuck in an infinite loop if something goes wrong.

See the commit message also.

--
jian
https://www.enterprisedb.com/