[C++] Implement optimizations for dictionary unification where dictionaries are prefixes of the unified dictionary #20688
Description
In the event that the unified dictionary contains other dictionaries as prefixes (e.g. as the result of delta dictionaries), we can avoid memory allocation and index transposition.
See discussion at
Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou
PRs and other links:
Note: This issue was originally created as ARROW-4095. Please see the migration documentation for further details.