terminology - What does it mean to vacuum a database? -

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What is the function of clearing the database? Is this operation supported by all modern database software or just a few people?

Databases that use to differentiate transactions from each other to periodic rows of old The tables need to be periodically scanned to remove copies. In MVCC, when a line is updated or removed, it can not be recycled immediately, because there can be active transactions, which can still see the old version of the row. If this is the case, instead of checking it, which is Can be quite expensive, old rows are considered to be relevant. The process of retrieving space becomes postponed until the table is blank, which can be started automatically or explicitly, depending on the database.


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