Add information about The Art Of PostgreSQL training

doc/development/database/database_reviewer_guidelines.md

@@ -62,6 +62,9 @@ The following guides provide a quick introduction and links to follow on more ad
 - Guide on [understanding EXPLAIN plans](../understanding_explain_plans.md).
 - [Explaining the unexplainable series in `depesz`](https://www.depesz.com/tag/unexplainable/).
 
+We also have licensed access to The Art of PostgreSQL available, if you are interested in getting access please check out the
+[issue (confidential)](https://gitlab.com/gitlab-org/database-team/team-tasks/-/issues/23).
+
 Finally, you can find various guides in the [Database guides](index.md) page that cover more specific
 topics and use cases. The most frequently required during database reviewing are the following:
 

doc/development/database_review.md

@@ -150,7 +150,7 @@ test its execution using `CREATE INDEX CONCURRENTLY` in the `#database-lab` Slac
 - Provide a public link to the plan from either:
   - [postgres.ai](https://postgres.ai/): Follow the link in `#database-lab` and generate a shareable, public link
     by clicking the **Share** button in the upper right corner.
-  - [explain.depesz.com](https://explain.depesz.com): Paste both the plan and the query used in the form.
+  - [explain.depesz.com](https://explain.depesz.com) or [explain.dalibo.com](https://explain.dalibo.com): Paste both the plan and the query used in the form.
 - When providing query plans, make sure it hits enough data:
   - You can use a GitLab production replica to test your queries on a large scale,
   through the `#database-lab` Slack channel or through [ChatOps](understanding_explain_plans.md#chatops).

doc/development/understanding_explain_plans.md

@@ -825,3 +825,5 @@ For more information about the available options, run:
 A more extensive guide on understanding query plans can be found in
 the [presentation](https://public.dalibo.com/exports/conferences/_archives/_2012/201211_explain/understanding_explain.pdf)
 from [Dalibo.org](https://www.dalibo.com/en/).
+
+Depesz's blog also has a good [section](https://www.depesz.com/tag/unexplainable) dedicated to query plans.