Adding a define-after-update will cause 2x the query count

[crisptrutski]

Found this in Metabase, where I need to add an after-update hook to :model/DashboardCard.

To reproduce, run the following test: https://github.com/metabase/metabase/blob/1f07e88cb40cc74813cdeedad205d11d077b5c84/test/metabase/models/dashboard_card_test.clj#L213

When run without an update hook, there are 10 / 11 queries.

If you then add the following no-op hook:

(t2/define-after-update :model/DashboardCard
  [instance]
  (prn "hook called for: " (:id instance))
  ;; we could also return instance, it does not make a difference.
  nil)

There will now be 16 / 17 queries. The hook will be called exactly 3 times, for the 3 distinct dashcards.

The extra queries are 3 pairs of extra SELECT and UPDATE statements.

On reflection this perhaps makes sense - we would need to query the db again to get default values and function results like NOW, but this would only explain the SELECT and not the extra UPDATE. That defo seems like a buggo.

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In my particular case I need to add a new hook to a large number of models, and the query amplification could be a large problem. What I need is a way to perform side effects using the same data that went to the before-hook. In fact, all I need to know are which fields changed (and don't care about the values). I do however need the transaction to have committed already, as my hook will trigger a database read from another thread.

Do you have any ideas on how I can do this without causing a large perf regression?

[crisptrutski]

Using toucan monitor:

Before

query	params
SELECT * FROM "report_dashboardcard" WHERE "id" = ?	[156]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 156]
DELETE FROM "dashboardcard_series" WHERE "dashboardcard_id" IN (?)	[156]
INSERT INTO "dashboardcard_series" ("dashboardcard_id", "card_id", "position") VALUES (?, ?, ?)	[156 638 0]
SELECT * FROM "report_dashboardcard" WHERE "id" = ?	[157]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 157]
DELETE FROM "dashboardcard_series" WHERE "dashboardcard_id" IN (?)	[157]
INSERT INTO "dashboardcard_series" ("dashboardcard_id", "card_id", "position") VALUES (?, ?, ?)	[157 639 0]
SELECT * FROM "report_dashboardcard" WHERE "id" = ?	[158]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 158]

After

query	params
SELECT * FROM "report_dashboardcard" WHERE "id" = ?	[168]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 168]
SELECT * FROM "report_dashboardcard" WHERE "id" IN (?)	[168]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 168]
DELETE FROM "dashboardcard_series" WHERE "dashboardcard_id" IN (?)	[168]
INSERT INTO "dashboardcard_series" ("dashboardcard_id", "card_id", "position") VALUES (?, ?, ?)	[168 650 0]
SELECT * FROM "report_dashboardcard" WHERE "id" = ?	[169]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 169]
SELECT * FROM "report_dashboardcard" WHERE "id" IN (?)	[169]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 169]
DELETE FROM "dashboardcard_series" WHERE "dashboardcard_id" IN (?)	[169]
INSERT INTO "dashboardcard_series" ("dashboardcard_id", "card_id", "position") VALUES (?, ?, ?)	[169 651 0]
SELECT * FROM "report_dashboardcard" WHERE "id" = ?	[170]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 170]
SELECT * FROM "report_dashboardcard" WHERE "id" IN (?)	[170]
UPDATE "report_dashboardcard" SET "size_x" = ?, "updated_at" = NOW(), "col" = ?, "row" = ? WHERE "id" = ?	[3 2 1 170]

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