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MySQL 5.1 Plugin Development by Andrew Hutchings, Sergei Golubchik

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Rows and keys

Now we discuss the row and key formats. The naive approach would be to store rows in a MySQL format—what is passed to the write_row() function is written to disk. Thus retrieval would be easy too. Unfortunately, this does not work. For example, this format has all CHAR and VARCHAR fields space padded to their full length. Storing them that way would waste a lot of space. But the main reason why this simple approach is fundamentally flawed is the BLOB type. In memory, all BLOB (and TEXT) fields are kept outside of the row buffer; the row buffer only keeps a pointer to the actual BLOB field content. This is why we use a special packed row format, which makes our code only slightly more complex, as MySQL conveniently provides a Field::pack() ...

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