pkolaczk commented on code in PR #3054:
URL: https://github.com/apache/cassandra/pull/3054#discussion_r1466706005
##########
src/java/org/apache/cassandra/index/sai/iterators/KeyRangeIntersectionIterator.java:
##########
@@ -77,7 +77,19 @@ protected PrimaryKey computeNext()
if (index != alreadyAvanced)
{
KeyRangeIterator range = ranges.get(index);
- PrimaryKey nextKey = nextOrNull(range, highestKey);
+ PrimaryKey nextKey = range.getCurrent();
+
+ // Note that we will either have a data model that
produces SKINNY primary keys or a data model
+ // that produces some combination of WIDE and STATIC
prikary keys.
+ if (nextKey.kind() == PrimaryKey.Kind.WIDE ||
nextKey.kind() == highestKey.kind())
+ // We can always skip if the target is of the same
kind or this range is non-static.
+ nextKey = nextOrNull(range, highestKey);
+ else if (nextKey.kind() == PrimaryKey.Kind.STATIC &&
nextKey.compareTo(highestKey) < 0)
+ // For a range of static keys, only skip if we'e
advanced to a new partition, and when we
+ // do, skip to an actual static key. We may otherwise
skip too far, as static row IDs always
+ // precede non-static ones in on-disk postings lists.
+ nextKey = nextOrNull(range, highestKey.toStatic());
Review Comment:
It is not very obvious to me it always terminates. This `if / else if` has a
theoretical path where it does *not* advance the range at all.
As I understood this fix, we don't want to advance a STATIC key until all
non-static keys were advanced - is it correct? A static key matches the whole
partition, so it must intersect with all the wide keys from another range, and
can be advanced only *after* those were used and reported as the result. It is
*probably* correct, but very, very non obvious and requires a lot of additional
knowledge (like e.g. assuming there can be either only WIDE/STATIC or SKINNY
but never mixed).
So maybe instead of doing such complex conditions (I am really staring at it
for 30 minutes and trying to understand the logic, constantly switching between
"Oh I get it" and "Oh no, I don't") could you split the loop into two loops -
one going over non-static keys and trying to advance them, and then another one
trying to advance the static ones (only if all non-static agree and the
highestkey jumped over the static ones) ?
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