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select columns from an insert

See original GitHub issue

Feature request

Is your feature request related to a problem? Please describe.

I need a way to perform an upsert, and know if I performed an update or an insert afterwards. Postgrest has an open issue to support this via 200 and 201 status codes https://github.com/PostgREST/postgrest/issues/1070. For the time being though, I can infer this on the client side using the xmax column. E.g.

POST http://<supabase-db-url>.supabase.co/rest/v1/items?select=id,xmax

If xmax === "0" then the row was inserted, otherwise an update occurred. Unfortunately, there is no way to write this kind of query with the supabase client.

Describe the solution you’d like

const result = await supabase
  .from('items')
  .insert([row], { onConflict: 'unique_field', upsert: true })
  .select('id,xmax')
if (result.data[0].xmax === "0") {
  console.log('insert')
} else {
  console.log('update')
}

Describe alternatives you’ve considered

write an rpc function to wrap this logic

Additional context

an issue on the PostgREST repo discussing possible solutions here: https://github.com/PostgREST/postgrest/issues/1683

selects on insert is valuable for multiple reasons outside of my outlined use case. Any performant client will be interested in returning the minimal rows necessary after an update/insert, rather than all of them all the time. I feel like I saw an issue or discussion around the default behavior around the RETURNING clause but for the life of me I cannot find it.