From: Michael Widenius
Date: April 8 2009 3:31pm
Subject: re: Deep copy of Item
List-Archive: http://lists.mysql.com/internals/36518
Message-Id: <18908.49988.892912.788814@narttu.askmonty.org>
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Hi!

>>>>> "Patrick" == Patrick Lau <patro02@stripped> writes:

Patrick> Hi
Patrick> For our purpose we want to free Items during query execution.

Patrick> So my idea would be:

Patrick> 1) instantiate item with malloc:

Patrick> Item* tmp_item = (Item*)malloc(sizeof(Item));

Patrick> 2) invoke copy constructor
Patrick> List_iterator_fast<Item> it(*join->fields);
Patrick> item1=it++;
tmp_item-> Item(*item1);

May not work in the future as we are likely to prohibit the usage of
copy constructors (as this leads to error with normal usage of items).

Patrick> now the question:
Patrick> - how would the copy constructor Item(& item) look like - especially
Patrick> for join->result->send_data(...)....
Patrick> - Is it a good idea?

Probably not.

Patrick> Does a similiar method already exist?

A better way would be to create a separate 'arena' for your items that
you then free all at once with the free_items() call.

This way you can free a group of items at once, any time during the
execution.

Look at the Query_arena() class in sql_class.cc

This is how we ensure that item's in prepared statements that we don't
want to free at the same time as the items used when executing the query.

Regards,
Monty

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