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Kenneth Reitz
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48 lines
2.1 KiB
ReStructuredText
`|Redis Documentation| <index.html>`_
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**Sets: Contents**
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`Redis Set Type <#Redis%20Set%20Type>`_
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`Implementation details <#Implementation%20details>`_
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Sets
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====
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#sidebar `SetCommandsSidebar <SetCommandsSidebar.html>`_
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Redis Set Type
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==============
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Redis Sets are unordered collections of
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`Redis Strings <Strings.html>`_. It's possible to add, remove, and
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test for existence of members in O(1).
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Redis Sets have the desirable property of not allowing repeated
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members. Adding the same element multiple times will result in a
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set having a single copy of this element. Practically speaking this
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means that adding an members does not require a "check if exists
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then add" operation.
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Commands operating on sets try to make a good use of the return
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value in order to signal the application about previous existence
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of members. For instance the `SADD <SaddCommand.html>`_ command
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will return 1 if the element added was not already a member of the
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set, otherwise will return 0.
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The max number of members in a set is 232-1 (4294967295, more than
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4 billion of members per set).
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Redis Sets support a wide range of operations, like union,
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intersection, difference. Intersection is optimized in order to
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perform the smallest number of lookups. For instance if you try to
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intersect a 10000 members set with a 2 members set Redis will
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iterate the 2 members set testing for members existence in the
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other set, performing 2 lookups instead of 10000.
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Implementation details
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======================
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Redis Sets are implemented using hash tables, so adding, removing
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and testing for members is O(1) in the average. The hash table will
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automatically resize when new elements are added or removed into a
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Set.
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The hash table resizing is a blocking operation performed
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synchronously so working with huge sets (consisting of many
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millions of elements) care should be taken when mass-inserting a
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very big amount of elements in a Set while other clients are
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querying Redis at high speed.
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It is possible that in the near future Redis will switch to skip
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lists (already used in sorted sets) in order to avoid such a
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problem.
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.. |Redis Documentation| image:: redis.png |