Thursday, January 29, 2009

NULL fields in Mysql row format

As a storage engine developer you need to take rows of data given by Mysql and interpret the meaning. You also need to take data from your storage engine format and convert this to Mysql format before returning the data to the engine. One important aspect of this is determining which fields have NULL values. Here is how the NULL values work in Mysql internal row format:

There is a table object that is part of the storage engine handler class:

struct st_table *table;

This table object contains a field member, which is an array of fields in the table:

Field **field;

Any individual field within a table has the following members:

uchar* null_ptr;
uchar null_bit;

The null_ptr is used to reference the byte location in the mysql row where the null bit mask for this field is located.

The null_bit is a bit mask for a single byte that can be used to set or test the NULL value for this field. So for example if you want to test the value of a specific field to see if it is NULL, you do it like this:

if (table->field[0]->null_ptr & table->field[0]->null_bit){
// field 0 is null
}

If you want to set a field to be null you can do it like this:

table->field[0]->null_ptr |= table->field[0]->null_bit;

Hope this helps.

Ivan Novick

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