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Displaying posts with tag: innodb dictionary (reset)

Recover Table Structure From InnoDB Dictionary
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When a table gets dropped MySQL removes respective .frm file. This post explain how to recover table structure if the table was dropped.

You need the table structure to recover a dropped table from InnoDB tablespace. The B+tree structure of InnoDB index doesn’t contain any information about field types. MySQL needs to know that in order to access records of InnoDB table. Normally MySQL gets the table structure from .frm file. But when MySQL drops a table the respective frm file removed too.

Fortunately there is one more place where MySQL keeps the tables structure . It is the InnoDB dictionary.

InnoDB dictionary is a set of tables where InnoDB keeps some information about the tables. I reviewed them in details is a separate InnoDB Dictionary post earlier. After the DROP InnoDB deletes

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Recover InnoDB dictionary
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Why do we need to recover InnoDB dictionary

c_parser is a tool from TwinDB recovery toolkit that can read InnoDB page and fetch records out of it. Although it can scan any stream of bytes recovery quality is higher when you feed c_parser with pages that belong to the PRIMARY index of the table. All InnoDB indexes have their identifiers a.k.a. index_id. The InnoDB dictionary stores correspondence between table name and index_id. That would be reason number one.

Another reason – it is possible to recover table structure from the InnoDB dictionary. When a table is dropped MySQL deletes respective .frm file. If you had neither backups nor table schema it becomes quite a challenge to recover the table structure. This topic however deserves a separate post which I write some other day.

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InnoDB dictionary
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Why Does InnoDB Need Dictionary

InnoDB dictionary is a set of internal tables InnoDB uses to maintain various information about user tables. It serves as API between a human and the database. While the humans refer tables by their names, InnoDB works with integer identifiers. The dictionary stores correspondence between table name and index_id.

The dictionary tables are normal InnoDB tables, but they’re not visible for a user. However some versions of MySQL provide read-only access to the dictionary in information_schema database.

The dictionary is stored in ibdata1. Root page of SYS_TABLES, for example, has id 8, so it’s eighth page from the beginning of ibdata1.

The dictionary pages are in REDUNDANT format even if you use MySQL 5.6. More about record formats I will write in future posts, I hope. For

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