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this key does not stop" (version for 5.0 only). UPDATE statement which WHERE clause used key and which invoked trigger that modified field in this key worked indefinetely. This problem occured because in cases when UPDATE statement was executed in update-on-the-fly mode (in which row is updated right during evaluation of select for WHERE clause) the new version of the row became visible to select representing WHERE clause and was updated again and again. We already solve this problem for UPDATE statements which does not invoke triggers by detecting the fact that we are going to update field in key used for scanning and performing update in two steps, during the first step we gather information about the rows to be updated and then doing actual updates. We also do this for MULTI-UPDATE and in its case we even detect situation when such fields are updated in triggers (actually we simply assume that we always update fields used in key if we have before update trigger). The fix simply extends this check which is done in check_if_key_used()/ QUICK_SELECT_I::check_if_keys_used() routine/method in such way that it also detects cases when field used in key is updated in trigger. As nice side-effect we have more precise and thus more optimal perfomance-wise check for the MULTI-UPDATE. Also check_if_key_used()/QUICK_SELECT_I::check_if_keys_used() were renamed to is_key_used()/QUICK_SELECT_I::is_keys_used() in order to better reflect that boolean predicate. Note that this check is implemented in much more elegant way in 5.1
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