Dyntex.net Support Centre: MySQL Reference Manual p.501

<< previous page -- table of contents -- next page >>

Chapter_1

Chapter_2

Chapter_3

Chapter_4

Chapter_5

Return to Table of Contents

Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

Chapter 7: MySQL Table Types 501 The rst time a table is updated in MySQL, a counter in the header of the index les is incremented. The counter is not changed during further updates. When the last instance of a table is closed (because of a FLUSH or because there isn't room in the table cache) the counter is decremented if the table has been updated at any point. When you repair the table or check the table and it was okay, the counter is reset to 0. To avoid problems with interaction with other processes that may do a check on the table, the counter is not decremented on close if it was 0. In other words, the only ways this can go out of sync are: The MyISAM tables are copied without a LOCK and FLUSH TABLES. MySQL has crashed between an update and the nal close. (Note that the table may still be okay, as MySQL always issues writes for everything between each statement.) Someone has done a myisamchk --repair or myisamchk --update-stateon a table that was in use by mysqld. Many mysqld servers are using the table and one has done a REPAIR or CHECK of the table while it was in use by another server. In this setup the CHECK is safe to do (even if you will get the warning from other servers), but REPAIR should be avoided as it currently replaces the datale with a new one, which is not signaled to the other servers. 7.2 MERGE Tables MERGE tables are new in MySQL Version 3.23.25. The code is still in gamma, but should be resonable stable. A MERGE table (also known as a MRG_MyISAM table) is a collection of identical MyISAM tables that can be used as one. You can only SELECT, DELETE, and UPDATE from the collection of tables. If you DROP the MERGE table, you are only dropping the MERGE specication. Note that DELETE FROM merge_table used without a WHERE will only clear the mapping for the table, not delete everything in the mapped tables. (We plan to x this in 4.1). With identical tables we mean that all tables are created with identical column and key information. You can't merge tables in which the columns are packed dierently, doesn't have exactly the same columns, or have the keys in dierent order. However, some of the tables can be compressed with myisampack. See Section 4.7.4 [myisampack], page 279. When you create a MERGE table, you will get a `.frm' table denition le and a `.MRG' table list le. The `.MRG' just contains a list of the index les (`.MYI' les) that should be used as one. All used tables must be in the same database as the MERGE table itself. For the moment, you need to have SELECT, UPDATE, and DELETE privileges on the tables you map to a MERGE table. MERGE tables can help you solve the following problems: Easily manage a set of log tables. For example, you can put data from dierent months into separate les, compress some of them with myisampack, and then create a MERGE to use these as one.

MySQL Reference Manual

Our Partners

SpiritHost - web hosting for spiritual and education sites
SpiritHit.com - Religious and Spiritual Portal

Return to Table of Contents	Back to top

Web Hosting: Manuals & FAQ's

1. Unix-Based Web Hosting
2. Unix Dedicated Servers
3. Windows Dedicated Servers
4. CuteFTP User’s Guide
5. CuteHTML User’s Guide
6. WS_FTP Pro User's Guide
7. Miva Order User's Guide
8. Miva Merchant User's Guide