Re: Optimizing execution of expensive subqueries

Hi pgsql-general!

I'm currently looking at a query that is generally selecting a bunch of simple columns from a table, and also performing some subqueries to aggregate related data, and then sorting by one of the simple columns and paginating the result.

eg. 

SELECT

  tbl.field1, tbl.field2, tbl.field3, ...,

  (SELECT SUM(Duration) FROM anothertbl WHERE anothertbl.UserId = tbl.UserId AND anothertbl.ThingyId = 1) as Thingy1Sum,

  ... repeat for multiply thingies ...

FROM

  tbl

ORDER BY tbl.field1 LIMIT 20

I'm finding that if "tbl" contains hundreds of thousands of rows, the subqueries are being executed hundreds of thousands of times.  Because of the sorting and pagination, this is appears to be unnecessary, and the result is slow performance.  (PostgreSQL 9.5.9 server)

I've only found one solution so far, which is to perform the sort & pagination in a CTE, and the subqueries externally.  Are there any other approaches that can be taken to optimize this and prevent the unnecessary computation?

CTE rewrite:

WITH cte AS (

SELECT

  tbl.field1, tbl.field2, tbl.field3

FROM

  tbl

ORDER BY tbl.field1 LIMIT 20

)

SELECT cte.*,

  (SELECT SUM(Duration) FROM anothertbl WHERE anothertbl.UserId = tbl.UserId AND anothertbl.ThingyId = 1) as Thingy1Sum,

  ... repeat for multiply thingies ...

FROM cte;

Thanks for any thoughts you have,

Mathieu Fenniak