1 May 2017

PostgreSQL: ERROR: argument of WHERE must not return a set

In my last post I showed how to load and query data from the Strava API in PostgreSQL and after executing some simple queries my next task was to query more complex part of the JSON structure.

Strava allows users to create segments, which are edited portions of road or trail where athletes can compete for time.

I wanted to write a query to find all the times that I’d run a particular segment. e.g. the Akerman Road segment covers a road running North to South in Kennington/Stockwell in South London.

This segment has the id '6818475' so we’ll need to look inside segment_efforts and then compare the value segment.id against this id.

I initially wrote this query to try and find the times I’d run this segment:

SELECT id, data->'start_date' AS startDate, data->'average_speed' AS averageSpeed
FROM runs
WHERE jsonb_array_elements(data->'segment_efforts')->'segment'->>'id' = '6818475'

ERROR:  argument of WHERE must not return a set
LINE 3: WHERE jsonb_array_elements(data->'segment_efforts')->'segmen...

This doesn’t work since https://www.postgresql.org/docs/9.5/static/functions-json.html#FUNCTIONS-JSON-PROCESSING-TABLE returns a set of boolean values, as Craig Ringer points out on Stack Overflow.

Instead we can use a LATERAL subquery to achieve our goal:

SELECT id, data->'start_date' AS startDate, data->'average_speed' AS averageSpeed
FROM runs r,
LATERAL jsonb_array_elements(r.data->'segment_efforts') segment
WHERE segment ->'segment'->>'id' = '6818475'

    id     |       startdate        | averagespeed
-----------+------------------------+--------------
 455461182 | "2015-12-24T11:20:26Z" | 2.841
 440088621 | "2015-11-27T06:10:42Z" | 2.975
 407930503 | "2015-10-07T05:18:34Z" | 2.985
 317170464 | "2015-06-03T04:44:59Z" | 2.842
 312629236 | "2015-05-27T04:46:33Z" | 2.857
 277786711 | "2015-04-02T05:25:59Z" | 2.408
 226351235 | "2014-12-05T07:59:15Z" | 2.803
 225073326 | "2014-12-01T06:15:21Z" | 2.929
 224287690 | "2014-11-29T09:02:46Z" | 3.087
 223964715 | "2014-11-28T06:18:29Z" | 2.844
(10 rows)

Perfect!

About the author

I'm currently working on short form content at ClickHouse. I publish short 5 minute videos showing how to solve data problems on YouTube @LearnDataWithMark. I previously worked on graph analytics at Neo4j, where I also co-authored the O'Reilly Graph Algorithms Book with Amy Hodler.