精确值与全文

    Exact values are exactly what they sound like. Examples would be a date or a
    user ID, but can also include exact strings like a username or an email
    address. The exact value is not the same as the exact value "foo".
    The exact value 2014 is not the same as the exact value 2014-09-15.

    Full text, on the other hand, refers to textual data — usually written in
    some human language — like the text of a tweet or the body of an email.

    Full text is often referred to as ``unstructured data’’, which is a misnomer
    — natural language is highly structured. The problem is that the rules of
    natural languages are complex which makes them difficult for computers to
    parse correctly. For instance, consider this sentence:

    Exact values are easy to query. The decision is binary — a value either
    matches the query, or it doesn’t. This kind of query is easy to express with
    SQL:

    1. WHERE name    = "John Smith"
    2.   AND date    > "2014-09-15"

    Querying full text data is much more subtle. We are not just asking How well does this document match the
    query?’’ In other words, how relevant is this document to the given query?

    We seldom want to match the whole full text field exactly. Instead, we want
    to search within text fields. Not only that, but we expect search to
    understand our intent:

    • a search for "jump" should also match "jumped", "jumps", "jumping"
      and perhaps even

    • "fox news hunting" should return stories about hunting on Fox News,
      while "fox hunting news" should return news stories about fox hunting.