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This bug is consequence of a serious architectural flaw of the query processing at the prepare stage. Context analysis of processed queries has to be done at this stage. The important part of this analysis is resolution of column references also known as name resolution. The items for column references used in a query are created by the parser. They are allocated in the statement memory because their life span is the same as the life span of the executed query. During name resolution some of the items are wrapped into objects belonging to classes derived from the Item_ref class. In many cases we can't do without such wrappers. For example, the parser does not differentiate between references to columns of base tables and references to columns of views. However for a reference to a view column we need a pointer to the translation item for this column. This pointer is stored in a Item_direct_view_ref object that wraps the item for the column reference. Before this patch the wrappers were allocated in execution memory good only for one execution. That wasn't a problem when the query was executed only once. Yet for the queries executed as prepared statements or within a stored procedure it could lead to crashes or wrong result sets at the second execution of the query. It could happen when the wrapped item was substituted for something else by permanent transformations during the optimization phase. This patch allocates all the wrappers around items created for column references in the statement memory at the first execution of PS or at the first call of SP/SF. |
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