The MonotonicFunction enum seems like a good pattern to follow here.
Learning about the existence of that now
typedef enum MonotonicFunction
{
MONOTONICFUNC_NONE = 0,
MONOTONICFUNC_INCREASING = (1 << 0),
MONOTONICFUNC_DECREASING = (1 << 1),
MONOTONICFUNC_BOTH = MONOTONICFUNC_INCREASING | MONOTONICFUNC_DECREASING,
} MonotonicFunction;
So, BOTH means that the function is both increasing and decreasing, thus it is constant.
Simply replacing the SLOPE_* by the corresponding MONOTONICFUNC_* does the job.
But now using an enum we will have 4 bytes per argument.
Sorry, I wasn't suggesting that you use MonotonicFunction outright, only that you follow it's pattern (make an enum, if you're needing specific bit patterns do it this way, etc. We often use enums as a way for switch statements to warn us when we're missing values.
So, how would this work with a function like left() with a positive 2nd param (assuming specified collation matches the index)?
That would require a separate prosupport function, it would have to do some extra work, checking if the second argument is a positive constant at planning time.
I'm happy to do that if there is interest, but I would keep after this.
I'm also curious if NULLS FIRST/LAST will throw this off.
All that this will do is, when considering index scans, check if the requested order
matches the index order. And whether we have to do a backwards or forwards scan.
What I meant was will we need a custom function to inspect parameters such that we *do* try the index optimization on substr(foo,x,..) if and only if x = 1?
It almost seems like what we need is "does f(x) preserve the leading sort bits of x, and reduce the trailing bits to ties?"
Is there a specialized nulls first/last sort or does it use a generic sort?
No, it just fed into my wondering about situations where the ordering of f(x) would be modified away from the ordering of x.
Oh, one other thing I am curious if any other reviewers like/dislike the
static const Slope pattern[2] = {SLOPE_ASC, SLOPE_DESC};
pattern. I haven't seen that one done before, and I'm wondering if we need some comments to explain how it works so that people don't start looking for the pfree that they'll never find.
