Reducing the number of rows returned

Although often the problem is producing many rows in the first place, it is made worse by returning all the unnecessary rows to the client. This is especially true if the client and server are not on the same host.

Here are some ways to reduce the traffic between the client and server.

How to do it…

Consider the following scenario: a full-text search returns 10,000 documents, but only the first 20 are displayed to users. In this case, order the documents by ranking on the server, and return only the top 20 that actually need to be displayed:

SELECT title, ts_rank_cd(body_tsv, query, 20) AS text_rank
  FROM articles, plainto_tsquery('spicy potatoes') AS query
  WHERE body_tsv @@ query
  ORDER BY rank DESC
  LIMIT 20
;

If you need the next 20 documents, don't just query with a limit of 40 and throw away the first 20. Instead, use OFFSET 20 LIMIT 20 to return the next 20 documents.

To gain some stability so that the documents with the same rank still come out...