Support 'prefix_extractor' option?

[webbery]

I look at the source code and find none about 'prefix_extractor'.
Will rocksdb support 'prefix_extractor' option?

[vweevers]

A PR is welcome. I don't know what prefix_extractor is and what kind of API it would require, so it'd be best to discuss that before opening a PR, so that we can assess the complexity.

[MeirionHughes]

From the looks of things there are three options, via classes:

• FixedPrefixTransform takes integer opt
• CappedPrefixTransform takes integer opt
• SamePrefixTransform takes string opt

Not sure whether SamePrefixTransform is enabled - its hidden behind a GFLAGS macro check, which is not defined according to my vscode. Its also in a test file - prefix_test.cc

presumably this could be exposed to the js side as

options: {
  ...
  prefix_extractor? : {
    fixed?: number,
    capped?: number,
    same?: string
  } 
}

where ? is optional notation and only 1 of them should be be defined.

on the binding side (db_open), it would require checking which is defined (if any), and mapping to the corresponding class;

i.e.

options.prefix_extractor.reset(NewFixedPrefixTransform(2));

before being sent to the open the db.

[vweevers]

What does it do?

[MeirionHughes]

Not 100% sure - but it seems it can optimize seeking/iterating when you're doing composite keys. https://github.com/facebook/rocksdb/wiki/Prefix-Seek#defining-a-prefix

[MeirionHughes]

I don't need this tbh - but I fancied having a gander at the code-base and tried to add it. #168