sql server - SQL Index - is this overlapping? -

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i'm going through , removing duplicate , redundant indexes in sql database.

so if have 2 identical indexes delete.

if have overlapping indexes delete, example...

  • index 1: brand, model
  • index 2: brand, model, price

i delete index 1.

what indexes in same order fields?

  • index 1: brand, model
  • index 2: brand, price, model

can safely delete index 1 or not?

my guess no i'm not sure

no, not safe remove that. order , position of columns important.

the introduction of price column between other 2 columns (brand , model) issue. without index on (brand,model), the model values "in order" under brand. price column added before model, model values no longer "in order".

for example, query that's doing this: 

where brand = 'foo'   , model >= 'bar' , model <= 'cat'

can make effective use of index on (brand,model)

it use index on (brand,price,model), range scan on leading column brand. after that, model values it's looking anywhere under that, under value of price, index less effective.

if don't have queries using index in (brand,model), safe drop it.

also, of these indexes may "unique" indexes uses enforce uniqueness, it's not safe remove those, unless there's equivalent index same columns in different order.


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