diff --git a/migrations/0002_schema.up.sql b/migrations/0002_schema.up.sql
new file mode 100644
index 0000000..cf279f8
--- /dev/null
+++ b/migrations/0002_schema.up.sql
@@ -0,0 +1,46 @@
+-- 0002_schema_up.sql
+
+-- ******************************************** --
+-- Change primary key of block table to height. --
+-- ******************************************** --
+
+-- Create new table
+CREATE TABLE IF NOT EXISTS block_new(
+    height INTEGER PRIMARY KEY NOT NULL,
+    hash TEXT NOT NULL
+);
+-- Copy only unique rows
+INSERT INTO block_new(height, hash)
+SELECT height, hash
+FROM block
+WHERE height IN(
+    SELECT height
+    FROM block
+    GROUP BY height
+    HAVING COUNT(*) = 1
+);
+-- Drop old table
+DROP TABLE block;
+-- Rename new table to old
+ALTER TABLE block_new RENAME TO block;
+
+-- ************************************************ --
+-- Change type of anchor block_hash column to TEXT. --
+-- ************************************************ --
+
+-- Create new table
+CREATE TABLE IF NOT EXISTS anchor_new(
+    block_height INTEGER NOT NULL,
+    block_hash TEXT NOT NULL,
+    txid TEXT NOT NULL,
+    confirmation_time INTEGER NOT NULL,
+    PRIMARY KEY(block_height, block_hash, txid)
+);
+-- Copy old data
+INSERT INTO anchor_new(block_height, block_hash, txid, confirmation_time)
+SELECT block_height, block_hash, txid, confirmation_time
+FROM anchor;
+-- Delete old table
+DROP TABLE anchor;
+-- Rename new table to old table
+ALTER TABLE anchor_new RENAME TO anchor;
diff --git a/src/async_store.rs b/src/async_store.rs
index 12a661e..dd1e5fa 100644
--- a/src/async_store.rs
+++ b/src/async_store.rs
@@ -140,20 +140,11 @@ impl Store {
         for (&height, hash) in &local_chain.blocks {
             match hash {
                 Some(hash) => {
-                    // Avoid inserting new rows of existing height.
-                    // FIXME: The correct way to handle this is to have a unique constraint on `height`
-                    // in the block table schema.
-                    let row_option = sqlx::query("SELECT height FROM block WHERE height = $1")
+                    sqlx::query("INSERT OR IGNORE INTO block(height, hash) VALUES($1, $2)")
                         .bind(height)
-                        .fetch_optional(&self.pool)
+                        .bind(hash.to_string())
+                        .execute(&self.pool)
                         .await?;
-                    if row_option.is_none() {
-                        sqlx::query("INSERT OR IGNORE INTO block(height, hash) VALUES($1, $2)")
-                            .bind(height)
-                            .bind(hash.to_string())
-                            .execute(&self.pool)
-                            .await?;
-                    }
                 }
                 None => {
                     sqlx::query("DELETE FROM block WHERE height = $1")