July 2010
Intermediate to advanced
840 pages
16h 33m
English
Content preview from Joe Celko's SQL for Smarties, 3rd Edition
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33.1. Access Methods
For this discussion, let us assume that there are four basic methods of getting to data: table scans or sequential reads of all the rows in the table, access via some kind of index, hashing, and bit vector indexes.
33.1.1. Sequential Access
The table scan is a sequential read of all the data in the order in which it appears in physical storage, grabbing one page of memory at a time. Most databases do not physically remove deleted rows, so a table can use a lot of physical space and yet hold little data. Depending on just how dynamic the database is, you may want to run a utility program to reclaim storage and compress the database. Performance can improve suddenly and drastically after database reorganization.
33.1.2. ...
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