schema: Add migration 0002_schema.up.sql

`0002_schema.up.sql` changes the PRIMARY KEY of `block`
table from `(height, hash)` to `height` only, in order
to prevent multiple rows from contending for the same height.

**NOTE** this operation is potentially destructive. If two
or more rows existed at the same height, they'll now be
missing from the database since we can't reliably determine
which block hash is the correct one.

Additionally, the anchor `block_hash` column previously
had the type INTEGER, and this was a mistake in the schema,
since block hashes are typically represented as a hex string.
Thus we change the type of the `block_hash` column to TEXT.

Author: ValuedMammal

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;