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Index Variations. Indexed Files - Part Three Analyzing the Options. Second Level Index in Multiple Files. Must have space to allow each 1:1 Index File to grow. Main index points to multiple secondary index files, which are each one cluster in size (e.g. 8KB). Sparse Second Level Index.

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index variations

Index Variations

Indexed Files - Part Three

Analyzing the Options

second level index in multiple files
Second Level Index in Multiple Files

Must have space to allow each

1:1 Index File to grow.

Main index points to multiple secondary index files, which are each one cluster in size (e.g. 8KB).

sparse second level index
Sparse Second Level Index
  • Q: Assuming 2-Levels, does the second level need to be 1:1?
  • A: It depends
    • if datafile is sorted, then no
    • if datafile is unsorted, then yes
multiple indexes for multiple keys
Multiple Indexes for Multiple Keys
  • If our key is Customer Name, then we cannot sort by Customer ID
    • please note that names are terrible keys, because names are not unique
multiple indexes for multiple views
Multiple Indexes for Multiple "Views"
  • Suppose there are different types of users, with different levels of access.
    • Implemented in Linux via "set effective user id".
three level index

Data File

Level Three

Three Level Index

Level Two

Level One

when to use 3 levels
When to Use 3 Levels
  • When the number of records in the datafile is very large.
  • Assume each index record for levels Two and Three take 28 bytes each
    • key = product ID = 20 bytes
    • RRN = long int = 8 bytes
  • BestMax Index section for internal sorting = 315 records
    • 8KB cluster / 28B = 315.01
  • If N = 1,000,000 data records
    • then level three contains 1,000,000 records
    • and level two contains 3175 index records
      • 1,000,000 / 315 = 3174.6
    • and level one contains 11 index records
timing analysis 1
Timing Analysis 1
  • Given a data file:
    • 100,000 records
    • no index
    • sorted
  • How much time does a searchtake on average?
  • Using a binary file search
    • log2N reads of datafile records
    • log2100,000 = 16.6
    • = 17 reads of datafile records
timing analysis 2
Timing Analysis 2
  • Given an Indexed File with
    • 100,000 records
    • two index levels
      • top level index size = 1,000 elements
  • How much time does a search take on average?
  • Level One
    • linear search time = N / 2
    • 1000 / 2 = 500 memory compares
  • Level Two
    • read the portion into memory = 1 read of 100 index records
    • binary search that portion = Log2 100 = 7 memory compares
  • Data File
    • 1 datafile record read
to index or not to index
To index, or not to index...
  • No Index File
    • 17 reads of datafile records
  • Two Levels of Index
    • 507 memory compares
    • 1 read of 100 index records
    • 1 datafile record read