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Asked: 2026-05-11T18:03:44+00:00

consider the following pgSQL statement: SELECT DISTINCT some_field FROM some_table WHERE some_field LIKE ‘text%’

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consider the following pgSQL statement:

SELECT DISTINCT some_field 
  FROM some_table 
  WHERE some_field LIKE 'text%' 
  LIMIT 10;

Consider also, that some_table consists of several million records, and that some_field has a b-tree index.

Why does the query take so long to execute (several minutes)? What I mean is, why doesnt it loop through creating the result set, and once it gets 10 of them, return the result? It looks like the execution time is the same, regardless of whether or not you include a ‘LIMIT 10’ or not.

Is this correct or am I missing something? Is there anything I can do to get it to return the first 10 results and ‘screw’ the rest?

UPDATE: If you drop the distinct, the results are returned virtually instantaneously. I do know however, that many of the some_table records are fairly unique already, and certianly when I run the query without he distinct declaration, the first 10 results are in fact unique. I also eliminated the where clause (eliminating it as a factor). So, my original question still remains, why isnt it terminating as soon as 10 matches are found?

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1 Answer

  1. Editorial Team

    You have a DISTINCT. This means that to find 10 distinct rows, it’s necessary to scan all rows that match the predicate until 10 different some_fields are found.

    Depending on your indices, the query optimizer may decide that scanning all rows is the best way to do this.

    10 distinct rows could represent 10, a million, an infinity of non-distinct rows.

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