title: Long db query result with concurrent write at the same time
body: <p>I'm curious. I have flow like that: We have big collection/table with a lot of data. And have some <code>select</code> oriented query which is long and takes 3 seconds.</p>
  
  <p>However we looking to highly concurrent environment and each seconds we get 100 new records in our database.</p>
  
  <p>So let's say we have a query, and before that query starts we have 1000 items which satisfy those query. Query takes 3 seconds, and each seconds there is 50 new items that match those query added to the db. My questions is - which is result this query return to me (is it still 1000 or 1150 or something in between) and how this depend from different database engine (SQL, NoSQL). O</p>
  
  <p>It's not a question about exact number, but a bit more - why it will be those number.</p>
  
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  <p>Looks like question is a bit wide. Let's limit DB with MySQL, Postgres, MongoDB and Cassandra. </p>