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During upgrades to IMSMA v6 (especially in Afghanistan), it has been noted that although the translation of MySQL queries/views into pgSQL queries/views (for details see [[Difference_MySQL_PostGreSQL|here]]) is relatively easy, sometimes views have to be rewritten due to poor performance. This is due to the fact that in each database system (e.g. MySQL vs. PostgreSQL vs. SQL Server) the query optimizer works a bit differently, and therefore a query that is fast in one system, might be very slow in another system.
Generally, the query optimizer has to make the following decisions:
* Scan method: e.g. sequential scan, (bitmap) index scan