Download Access methods for time

Presenters : Virag Kothari ,Vandana Ayyalasomayajula
Date: 04/21/2010
Outline
Introduction to temporal databases
 Goal of the paper
 Access method costs
 Queries
 Index pagination & Data Clustering
 Efficient method design for transaction data
 References

Introduction

Based on time dimension
 Transaction time database
 Valid time database
 Bitemporal database
Goal
Attempt to identify the implications for access
method design from support of each time
dimension
 In this presentation, transaction time
databases are considered.

Access Method Costs

Performance of an access method depends on
 storage space to physically store the data
records and the structures of the access method
 update processing time (the time to update the
method’s data structures as a result of a change)
 the query time for each of the basic queries
( discussed in the next slide !)
Queries

Given a contiguous interval T, find all objects
alive during this interval.

Given a key range and a contiguous time
interval T, find the objects with keys in the
given range that are alive during interval T.

Given a key range, find the history of the
objects in this range.
Queries

- Special cases !
“transaction pure-timeslice”
 A special case of class (I) occurs when interval T is
reduced to a single transaction time instant t.

“transaction range-timeslice”
 representative case of class (II) where the time interval
is reduced to a single transaction time instant.

“transaction pure-key query”
 representative case of class (III), key range is reduced
to a single key
Cost parameters

In the case of transaction , Bitemporal
databases,
 n - > summation of insertions, deletions, and
modification updates.

For Valid time databases,
 L - > the number of interval objects currently stored in the
method, i.e., the size of the collection

a -> to denote the answer size of a query in
general.
Index pagination & Data
Clustering
Cost depends on IO cost !
 Performance of an index depends on how
well it is ‘Paginated’

 Example: B+ trees.

Data Clustering improves performance by
storing logically near data , physically close
on the disk.
 pure-timeslice query takes O(logBn + a/B )
page accesses. This method is more I/O
efficient than another method that solves the
same query in O(logBn + a) page accesses.
Efficient Method Design for
Transaction

Transaction Pure-Timeslice Query
 ‘copy’ approach
○ Stores a copy of the transaction database state s(t)
(timeslice) for each transaction time that at least one
change occurred
○ Copies are indexed by time t.
 ‘log’ approach
○ Stores only the changes that occur in the database
timestamped by the time instant on which they
occurred.
○ Copies indexed by time t.
Comparison
Space
Copy approach
Log approach
O( n^2/ B)
O(n/B)
– Transaction pure timeslice
Update
O(n/B)
O(1)
Query
O(logB n) +
O(a/B)
O(n/B)
Transaction Pure Key
“copy” and “log” solutions could be used for the
pure-key query. But they are both very inefficient
!!.
 A better solution is to store the history of each
key separately, i.e., cluster data by key only.
 Access to a key’s (transaction time) history can
be implemented by a hashing function or B tree.
 The list of versions of each key can be further
organized in a separate array indexed by
transaction time to answer a pure-key query with
time predicate .

Costs
– Transaction pure key
Cost to index into hash table or B tree +
cost of searching in the array.
 Array length can be n/B , so cost would
O( log B n) .

Transaction Range-Timeslice
To answer a range query efficiently, it is best to
cluster by transaction time and key within
pages.
 Very similar to spatial indexing concept.
 Two dimensions , time & key need to be
considered.
 Data bounding technique
 Another possibility data mapping, maps a
record to three (or more) coordinates –
transaction start_time, end-_time, and key(s)—
and then uses a multiattribute point index.

References

Betty Salzberg, Vassilis J. Tsotras:
Comparison of Access Methods for
Time-Evolving Data. ACM Comput. Surv.
(CSUR) 31(2):158-221 (1999)
Thank you!!