Add issue chains decision

docs/decisions/0004-issue-chains-in-the-db.md

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+# How to express issue chains in the database
+
+Author: @rsolovjov
+Date: 16.08.2019
+
+## Decision
+We will use Materialized Path with [PostgresSQL ltree](https://www.postgresql.org/docs/9.2/ltree.html)
+
+## Reasoning
+Candidates:
+1. Adjacency List
+2. Nested Set
+3. Materialized Path
+
+Materialized Path has advantages:
+1. The ltree implements a **materialized path,** which very quick for INSERT/UPDATE/DELETE and pretty quick for SELECT operations
+2. It will be generally faster than using a recursive CTE or recursive function that constantly needs to recalculate the branching
+4. As built-in query syntax and operators specifically designed for querying and navigating trees
+5. Using a GiST index with ltree operators allows Postgres to search efficiently across an entire tree
+
+Some libraries implementing Materialized Path exists:
+- [**ecto_materialized_path**](https://github.com/asiniy/ecto_materialized_path)
+- [**hierarch**](https://github.com/Byzanteam-Labs/hierarch)
+- [**ecto_ltree**](https://hexdocs.pm/ecto_ltree/0.2.0/readme.html)
+
+But they have limitations, for example, **hierarch** and **ecto_materialized_path** functions work only when path consists of **id** fields, i.e. we can't use any other fields like **iid**. Аnd anyway you have to use **fragment** to use the built-in query syntax and operators.
+And better just define your own Ecto LTree type without any libraries.
+
+## Implementation
+
+1. Define Ecto LTree type
+https://hostiledeveloper.com/2017/06/17/ltrees-in-phoenix.html
+
+2. Add "path" field to "issues" table and create a new table for implementing "after" feature.
+```
+alter table(:issues) do
+  add :path, :ltree
+end
+
+create table(:issue_after) do
+  add(:after_id, references(:issues))
+  add(:issue_id, references(:issues))
+end
+```
+3. Using the fragment function to get our [ltree search](https://www.postgresql.org/docs/9.2/ltree.html) working. For example:
+```
+def get_descendants(root_issue) do
+  from(issue in Issue,
+    where: fragment("? <@ ?", issue.path, ^root_issue.path))
+  |> Repo.all()
+end
+```