Integrating clickhouse for metrics.

priv/clickhouse/migrations/20260407000001_create_post_events.exs

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+defmodule Mixer.ClickhouseRepo.Migrations.CreatePostEvents do
+  use Ecto.Migration
+
+  @doc """
+  Creates the `post_events` table using a MergeTree engine.
+
+  Key design decisions:
+
+  * `LowCardinality(String)` for `event_type` — the cardinality is tiny
+    (5–10 values), so ClickHouse can store it as a dictionary, giving both
+    compression and faster filtering.
+
+  * `Nullable(UUID)` / `Nullable(String)` for optional columns — ClickHouse
+    handles NULLs differently from PostgreSQL; we make the nullable fields
+    explicit so the schema is unambiguous.
+
+  * `ORDER BY (occurred_at, event_type, tweet_id)` — optimises the two most
+    common query patterns:
+      1. Time-range scans (`WHERE occurred_at >= now() - interval 24 HOUR`)
+      2. Per-tweet aggregations (`WHERE tweet_id = ?`)
+
+  * `PARTITION BY toYYYYMM(occurred_at)` — monthly partitions make it cheap
+    to drop old data with `ALTER TABLE … DROP PARTITION`.
+
+  * `TTL occurred_at + INTERVAL 1 YEAR DELETE` — automatically reclaim disk
+    space after two years. Adjust as required.
+  """
+  def up do
+    execute("""
+    CREATE TABLE IF NOT EXISTS post_events
+    (
+        event_type  LowCardinality(String),
+        tweet_id    UUID,
+        user_id     Nullable(UUID),
+        occurred_at DateTime,
+        ip_address  Nullable(String)
+    )
+    ENGINE = MergeTree()
+    PARTITION BY toYYYYMM(occurred_at)
+    ORDER BY (occurred_at, event_type, tweet_id)
+    TTL occurred_at + INTERVAL 1 YEAR DELETE
+    SETTINGS index_granularity = 8192
+    """)
+  end
+
+  def down do
+    execute("DROP TABLE IF EXISTS post_events")
+  end
+end