Store and use quick-hashed values for reverse lookups

[rnewman]

A common pattern is to refer to records by some unique property:

[:find ?title :where [
  [[:page/url ?] :page/title ?title]
]]
; Binding: "http://foo.com/"

That involves matching string values of :page/url to find a page, then fanning back out to title, or matching all titles and pages and narrowing by URL.

Rather than doing string matching directly, it's often more efficient to store a known hash of a value, and query with a pattern like:

["… WHERE datoms.vh = ? AND datoms.v = ?" (quick-hash val) val]

We should optionally store these in the DB:

vh INTEGER       // Can be null

With a partial index:

CREATE INDEX idx_vh ON datoms (vh, v, p) WHERE vh IS NOT NULL;

and then add flags to opt in or out on a per-attribute level. We might choose to do this automatically for string-valued unique attributes like :page/url.

[ncalexan]

This is an interesting proposal. I like the last part: "automatic[ally] for string-valued unique attributes".

[rnewman]

Places just made a similar change, and IIRC achieved index size reductions (no URL strings in indices) and probably some speed ups.