PostgreSQL - repeating rows from LIMIT OFFSET

Question

I noticed some repeating rows in a paginated recordset.

When I run this query:

SELECT \"students\".* 
FROM \"students\" 
ORDER BY \"students\".\"stat         


        

Answer 1

For more details from PostgreSQL documentation (http://www.postgresql.org/docs/8.3/static/queries-limit.html) :

When using LIMIT, it is important to use an ORDER BY clause that constrains the result rows into a unique order. Otherwise you will get an unpredictable subset of the query's rows. You might be asking for the tenth through twentieth rows, but tenth through twentieth in what ordering? The ordering is unknown, unless you specified ORDER BY.

The query optimizer takes LIMIT into account when generating a query plan, so you are very likely to get different plans (yielding different row orders) depending on what you give for LIMIT and OFFSET. Thus, using different LIMIT/OFFSET values to select different subsets of a query result will give inconsistent results unless you enforce a predictable result ordering with ORDER BY. This is not a bug; it is an inherent consequence of the fact that SQL does not promise to deliver the results of a query in any particular order unless ORDER BY is used to constrain the order.