[FEA] Story - Supporting row operators on nested types

[devavret]

There have been several requests to enable row operators on nested types. This issue is to track all related issues as a story.

There are three types of row operators we need to support (equality comparison ==, lexicographic comparison <, and hashing #) on two different nested types (LIST and STRUCT).

Status	Issue	Type	Operation	Notes
✔️	#8683	STRUCT	==	Solved by flattening
NULL_MIN and NULL_MAX still outstanding, see #11520	#8964	STRUCT	==, <	Proposed solution by flattening #9452
waiting on cuDF list index support, see #8039	#8039	LIST	(== + #) / (== + <)	Requires == + # for hash groupby or == + < for sort groupby. Also a Spark req #10181
waiting on min/max struct in hash-groupby	#8974	STRUCT	<	Individual action items being solved with flattening
✔️	#5890	LIST	<	Plain list sorting. Spark req #10184
awaiting cuDF troubleshooting, see #6784	#6784	LIST	(== + #) / (== + <)	drop_duplicates uses (== + <) right now but will be optimized to use (== + #) in #10030
✔️	#9119	STRUCT	#	Needs a struct_device_view
✔️	#10378	LIST	#	We have list hashing, Spark-compatible Murmur3 hashing for lists
Proposal in #11222	#10408	LIST of STRUCT	<	Different from groupby on list because here the list<struct> column is values, not keys

The plan

This will be supported using multiple PRs, first covering 1-table row comparators and hashing for nested types, then extending the row comparators with 2-table versions:

PR	Column Type	Operation	Number of tables	Dependencies	Notes
#10164	Struct	<	1		Refactor of existing functionality. Introduces new owning operator API
#10289	List + struct (arbitrary nesting)	==	1	#10164, #10291	Works only for "sanitized list" and structs with nulls pushed down
#10641	List + struct (arbitrary nesting)	#	1	#10289	Same requirements as #10289
#10883	List + struct (arbitrary nesting)	==	2	#10289,	Same requirements as #10289, also see #10508
#10730	Struct	<	2	#10164	#10508
#11129	List	<	1	#10164, #10291	Should work for struct of list but won't work for list of structs
#11292	List	Spark-#	1
	List of struct	<	1		Proposal in #11222
	Struct of list	<	1		We expected #11129 would cover these types, but there is an unresolved issue

[devavret]

We would need to figure out how to pass column_device_view information as part of the object.

This may be achieved by specializing element_hasher for list_view and passing list_device_view to hash_function. Then instead of defining MurmurHash3_32<cudf::list_view> we can define MurmurHash3_32<cudf::list_device_view>

[github-actions]

This issue has been labeled inactive-30d due to no recent activity in the past 30 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed. This issue will be labeled inactive-90d if there is no activity in the next 60 days.