lec15 - Hash-Based Indexes + More Material from Chapter 11...

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1 3/15/11 EECS 484: Database Management Systems, Kristen LeFevre 1 Hash-Based Indexes + More Material from Chapter 11 3/15/11 EECS 484: Database Management Systems, Kristen LeFevre 2 Hash Indexes B+ Trees are the most common type of index in relational DBMSs, but there are other types of indexes, too! Indexes vary in terms of organization and functionality (i.e., the types of queries they help support) Today’s Example: Hash-based Indexes Helps support equality (not range) search
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2 3/15/11 EECS 484: Database Management Systems, Kristen LeFevre 3 Agenda • Static Hashing: A simple strawman; does not support incremental maintenance • Extendible Hashing: A more advanced incremental hash-based index Gracefully supports inserting and deleting data entries Linear Hashing: Another incremental hash- based index (see textbook for details) 3/15/11 EECS 484: Database Management Systems, Kristen LeFevre 4 Hash Tables (Brief Review) Recall common (in-memory) data structure from 281 Keys Buckets Hash Function h(key) ! Obj1 Obj3 Obj4 Obj2 Alice Bob Carlos Dave Buckets can be implemented as an array of pointers to linked lists of C++ objects What are important properties of the hash function h()?
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3 3/15/11 EECS 484: Database Management Systems, Kristen LeFevre 5 Hash Indexes Idea is similar to C++ hash tables, but with a new twist Index can be much larger than memory! Should store index on database pages, rather than in C++ data structures 3/15/11 EECS 484: Database Management Systems, Kristen LeFevre 6 Static Hashing Similar to hash table, but data entries (buckets) are stored on pages Primary Bucket Pages (each contains many data entries k*) 0 1 2 N-1 Overflow Pages h(key) ! mod N Key How do we search for a particular key value?
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4 3/15/11
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lec15 - Hash-Based Indexes + More Material from Chapter 11...

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