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When searching for other users with the same email address, we perform the case-insensitive and accent-sensitive search. That may be expensive as some DBs such as MySQL cannot use the index in that case. Instead, sequential scan of all the user records is performed and the comparison uses the LOWER function to filter the matching records. This leads to significant performance heavy queries which in turn represent a surface for DoS attacks. For that reason, we first perform accent-insensitive search for potential candidates in a subselect, which can use the index. Only then we perform the additional accent-sensitive search on this limited set or records.
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