Question & Answer
Question
What is Automatic Read-Ahead Please see links below for more information.
Answer
Automatic read-ahead operations help improve query performance by issuing asynchronous page requests when the database server detects that the query is encountering I/O. Asynchronous page requests can improve query performance by overlapping query processing with the processing
necessary to retrieve data from disk and put it in the buffer pool.
Generally, the default value of 1 is appropriate for most production
environments.
Automatic readahead is designed to alleviate the need to have to worry about configuring Informix to get optimal i/o readahead for queries. It works by turning readahead on and off during query execution whenever the data being read dictates what’s best for the query. It works for the 3 basic types of scans;
-- index (keyonly),
-- data (sequential)
-- index/data (retrieve rids from the index and look up the full row from the data partition).
As a query is scanning data, if Informix encounters the need to read a page in from disk, it will immediately start issuing readahead requests. If the query scans a significant number of pages and finds that they are already cached, Informix will stop issuing readahead requests for that query until it encounters another page needing to be read from disk.
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Modified date:
16 June 2018
UID
swg21682135