Reducing the number of rows returned
Although the problem often produces too many rows in the first place, it is made worse by returning all 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 rank 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
;
The ORDER BY clause ensures the rows are ranked, and then the LIMIT 20 returns only the top 20.
If you need the next 20 documents, don’t just query with a limit of 40 and throw away the first 20. Instead...