MARKLOGIC SERVER

For each range index, MarkLogic creates data structures that make it easy to do two
things: for any document ID, get the document's range index value(s); for any range
index value, get the document IDs that have that value.
Conceptually, you can think of a range index as implemented by two data structures,
written to disk and then memory mapped for efficient access. One can be thought of
as an array of structures holding document IDs and values, sorted by document IDs,
and the other an array of structures holding values and document IDs, sorted by values.
It's not actually this simple or wasteful with memory and disk (in reality, the values are
only stored once), but it's a good mental model. With our party example, you'd have a
list of birthdays mapped to people, sorted by birthday, and a list of people mapped to
birthdays, sorted by person.
RANGE QUERIES
To perform a fast range query, MarkLogic uses the "value to document ID" lookup
array. Because the lookup array is sorted by value, there's a specific subsequence in
the array that holds the values between the two user-defined endpoints. Each of the
values in the range has a document ID associated with it, and those document IDs
can be quickly gathered and used like a synthetic term list to limit the search result to
documents having a matching value within the user's specified range.
For example, to find partygoers with a birthday between January 1, 1980, and May
16, 1980, you'd find the point in the date-sorted range index for the start date, then
the end date. Every date in the array between those two endpoints is a birthday of
someone at the party, and it's easy to get the people's names because every birthdate has
the person listed right next to it. If multiple people have the same birthday, you'll have
multiple entries in the array with the same value but a different corresponding name. In
MarkLogic, instead of people's names, the system tracks document IDs.
Range queries can be combined with any other types of queries in MarkLogic. Let's
say that you want to limit results to those within a date range as above but also having
a certain metadata tag. MarkLogic uses the range index to get the set of document IDs
in the range, uses a term list to get the set of document IDs with the metadata tag, and
intersects the sets of IDs to determine the set of results matching both constraints. All
indexes in MarkLogic are fully composable with each other.
Programmers probably think they're using range indexes only via functions such as
cts:element-range-query(). In fact, range indexes are also used to accelerate
regular XPath and XQuery expressions. The XPath expression /book[metadata/
price > 19.99] looks for books above a certain price and will leverage a decimal
(or double or float) range index on if it exists. What if there's no such
range index? Then MarkLogic won't be able to use any index to assist with the price
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