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How Not to Perform a Difficult Update in SQL Server/Azure

I learned a long time ago, that the quickest way to do something was to not do it at all.

A friend of mine asked me if I could review an update statement that was confounding their group looking for ways to optimize it.

The table has, just short of 50 columns, a considerable number of indexes and the column in question, has a datatype set to NVarChar(8), consists of some numbers, combinations of letters and numbers, etc.

The process would take 7 ½ hours to run this update, as it parses through 100K rows at a time. There is one index that includes the CN1 column in it, but no index on just the CN1 column. This is what the update statement looks like: