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A splayed table on disk with an attribute on a column would be worth testing as these can be mapped rather than requiring to be all in memory
https://code.kx.com/q/ref/set-attribute/#unique
Using 1: to write an Anymap file also creates a mappable object worth exploring
https://code.kx.com/q/releases/ChangesIn3.6/#anymap
If a single splay/anymap would be too large a fixed size int partitioned DB on a fixed range hash of the alphanumeric string could be used