Why is postgresql update query so slow sometimes, even with index?

My first guess would be that you have some other query locking the whole table or the rows that are being updated. Your simple update is the forced to wait for that other operation to complete.

Check, that you don't have any index or constraint on the updated column. If so, then the database might be doing index recalculation or constraint evaluation. These additional tasks are not included in the EXPLAIN ANALYZE result.

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