sql - Why is like-operator so fast -

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you know operator in sql. example:

select *  customer  email '%goog%'

so question how can database return fast result? when should program function this, loop on customers , on each email. slow. heard indexes. how can database use index when database doesn't know first or last letter is? or other way it?

i don't want program this. want know how works.

i have no idea engine using , what's beneath actual hood here helpful information regarding problem:

  1. often, sql engines uses free text search inside column able extract queries fast. done creating inverted index, maps each word "documents" (row,column) contains them. 1 used library apache lucene. unfortunately, ir (information retrieval) libraries not support wild card @ beginning of query (but anywhere else), specific example cannot searched in such index.
  2. you can create index support wild card @ beginning of index, using suffix tree. suffix trees excellent searching substring, example. however, not optimized searching string wild card in middle of it.

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