diff --git a/internal/database/postgres/migrations/000109_recommendations_availability_zone.down.sql b/internal/database/postgres/migrations/000109_recommendations_availability_zone.down.sql
new file mode 100644
index 00000000..b9023887
--- /dev/null
+++ b/internal/database/postgres/migrations/000109_recommendations_availability_zone.down.sql
@@ -0,0 +1,4 @@
+DROP INDEX IF EXISTS recommendations_natural_key_az_idx;
+
+ALTER TABLE recommendations
+    DROP COLUMN IF EXISTS availability_zone;
diff --git a/internal/database/postgres/migrations/000109_recommendations_availability_zone.up.sql b/internal/database/postgres/migrations/000109_recommendations_availability_zone.up.sql
new file mode 100644
index 00000000..2160caf9
--- /dev/null
+++ b/internal/database/postgres/migrations/000109_recommendations_availability_zone.up.sql
@@ -0,0 +1,34 @@
+-- Migration 000109: availability_zone generated column and a 9-column unique
+-- index on recommendations (#792, PR2a of the zonal EC2 AZ work).
+--
+-- Zonal EC2 recommendations for the same instance type, term and payment
+-- differ only by availability zone, but the 8-column natural key cannot tell
+-- them apart. This migration adds the AZ to the schema; nothing uses it yet.
+--
+-- The column is derived from the stored payload (the JSON of the
+-- RecommendationRecord) so existing rows are backfilled by the table rewrite
+-- with no data migration: the top-level "availability_zone" field wins, and
+-- rows written before that field existed fall back to details.availability_zone
+-- (the ComputeDetails blob). Anything else, including every non-zonal row,
+-- becomes ''.
+--
+-- The existing 8-column recommendations_natural_key_idx is KEPT, so the
+-- current ON CONFLICT (account_key, provider, service, region, resource_type,
+-- engine, term, payment_option) upsert keeps working for old and new code. As a
+-- consequence two zonal rows that differ only by AZ are still rejected by that
+-- index until a follow-up migration replaces it (PR2c, together with the store
+-- change to the 9-column conflict target).
+--
+-- Lock impact: ADD COLUMN ... GENERATED ... STORED rewrites the table and the
+-- index build blocks writes, both under ACCESS EXCLUSIVE / SHARE locks for the
+-- duration. The table is a regenerable cache re-populated every collection
+-- cycle, so this is acceptable. CREATE INDEX CONCURRENTLY is not possible:
+-- the migration runner wraps every migration in a transaction.
+ALTER TABLE recommendations
+    ADD COLUMN IF NOT EXISTS availability_zone TEXT
+    GENERATED ALWAYS AS (
+        COALESCE(payload->>'availability_zone', payload->'details'->>'availability_zone', '')
+    ) STORED;
+
+CREATE UNIQUE INDEX IF NOT EXISTS recommendations_natural_key_az_idx
+    ON recommendations (account_key, provider, service, region, resource_type, engine, term, payment_option, availability_zone);
diff --git a/internal/database/postgres/migrations/000109_recommendations_availability_zone_test.go b/internal/database/postgres/migrations/000109_recommendations_availability_zone_test.go
new file mode 100644
index 00000000..b146da37
--- /dev/null
+++ b/internal/database/postgres/migrations/000109_recommendations_availability_zone_test.go
@@ -0,0 +1,128 @@
+//go:build integration
+// +build integration
+
+package migrations_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/migrations"
+	"github.com/LeanerCloud/cloud-commitments-platform/internal/database/postgres/testhelpers"
+	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const insertRecForMigration109 = `
+	INSERT INTO recommendations
+	    (collected_at, provider, service, region, resource_type, engine, payload,
+	     upfront_cost, monthly_savings, term, payment_option)
+	VALUES (NOW(), 'aws', 'ec2', 'us-east-1', $1, '', $2::jsonb, 0, 0, 1, 'all-upfront')
+`
+
+// TestMigration_RecommendationsAvailabilityZone locks down migration 000109
+// (issue #792, PR2a): the generated availability_zone column backfills from
+// both payload shapes, the 8-column index is kept so the current upsert still
+// works, and rollback removes the column and the new index.
+func TestMigration_RecommendationsAvailabilityZone(t *testing.T) {
+	ctx := context.Background()
+	migrationsPath := getMigrationsPath()
+
+	container, err := testhelpers.SetupPostgresContainer(ctx, t)
+	require.NoError(t, err)
+	defer container.Cleanup(ctx)
+	pool := container.DB.Pool()
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, 108))
+
+	// Rows as written before the migration, in every payload shape.
+	legacy := map[string]string{
+		// Written before the top-level field existed: AZ only inside Details.
+		"legacy-details-only": `{"details":{"instance_type":"m5.large","scope":"Availability Zone","availability_zone":"us-east-1a"}}`,
+		// Written by the scheduler since #807: top-level and Details agree.
+		"both-shapes": `{"availability_zone":"us-east-1b","details":{"instance_type":"m5.large","scope":"Availability Zone","availability_zone":"us-east-1b"}}`,
+		// Top-level only.
+		"top-level-only": `{"availability_zone":"us-east-1c"}`,
+		// Regional EC2 rec: Details present, no AZ.
+		"regional": `{"details":{"instance_type":"m5.large","scope":"Region"}}`,
+		// No details at all.
+		"no-details": `{"service":"ec2"}`,
+	}
+	for resourceType, payload := range legacy {
+		_, err = pool.Exec(ctx, insertRecForMigration109, resourceType, payload)
+		require.NoError(t, err)
+	}
+
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, 109))
+
+	want := map[string]string{
+		"legacy-details-only": "us-east-1a",
+		"both-shapes":         "us-east-1b",
+		"top-level-only":      "us-east-1c",
+		"regional":            "",
+		"no-details":          "",
+	}
+	rows, err := pool.Query(ctx, `SELECT resource_type, availability_zone FROM recommendations`)
+	require.NoError(t, err)
+	got := map[string]string{}
+	for rows.Next() {
+		var rt, az string
+		require.NoError(t, rows.Scan(&rt, &az))
+		got[rt] = az
+	}
+	require.NoError(t, rows.Err())
+	assert.Equal(t, want, got, "availability_zone must be backfilled from either payload shape and default to ''")
+
+	// Both indexes exist: the 8-column one is kept.
+	for _, idx := range []string{"recommendations_natural_key_idx", "recommendations_natural_key_az_idx"} {
+		var exists bool
+		require.NoError(t, pool.QueryRow(ctx,
+			`SELECT EXISTS (SELECT 1 FROM pg_indexes WHERE tablename = 'recommendations' AND indexname = $1)`, idx).Scan(&exists))
+		assert.True(t, exists, "index %s must exist after 000109", idx)
+	}
+
+	// The 9-column index is unique and ends with availability_zone. It cannot be
+	// exercised by inserts yet (the kept 8-column index rejects the same rows),
+	// so assert its definition.
+	var indexDef string
+	require.NoError(t, pool.QueryRow(ctx,
+		`SELECT indexdef FROM pg_indexes WHERE indexname = 'recommendations_natural_key_az_idx'`).Scan(&indexDef))
+	assert.Contains(t, indexDef, "UNIQUE INDEX")
+	assert.Contains(t, indexDef, "engine, term, payment_option, availability_zone)")
+
+	// Old code's upsert (8-column ON CONFLICT, no availability_zone) still works
+	// and updates the existing row in place.
+	upsert := `
+		INSERT INTO recommendations
+		    (collected_at, provider, service, region, resource_type, engine, payload,
+		     upfront_cost, monthly_savings, term, payment_option)
+		VALUES (NOW(), 'aws', 'ec2', 'us-east-1', 'both-shapes', '', $1::jsonb, 5, 0, 1, 'all-upfront')
+		ON CONFLICT (account_key, provider, service, region, resource_type, engine, term, payment_option)
+		DO UPDATE SET payload = EXCLUDED.payload, upfront_cost = EXCLUDED.upfront_cost`
+	_, err = pool.Exec(ctx, upsert, `{"availability_zone":"us-east-1b"}`)
+	require.NoError(t, err, "the existing 8-column upsert must keep working after 000109")
+	var n int
+	require.NoError(t, pool.QueryRow(ctx, `SELECT COUNT(*) FROM recommendations WHERE resource_type = 'both-shapes'`).Scan(&n))
+	assert.Equal(t, 1, n)
+
+	// Known limitation, resolved in PR2c: two zonal rows differing only by AZ
+	// are still rejected by the kept 8-column index.
+	_, err = pool.Exec(ctx, insertRecForMigration109, "zonal-pair", `{"availability_zone":"us-east-1a"}`)
+	require.NoError(t, err)
+	_, err = pool.Exec(ctx, insertRecForMigration109, "zonal-pair", `{"availability_zone":"us-east-1b"}`)
+	var pgErr *pgconn.PgError
+	require.ErrorAs(t, err, &pgErr)
+	assert.Equal(t, "23505", pgErr.Code)
+	assert.Equal(t, "recommendations_natural_key_idx", pgErr.ConstraintName)
+
+	// Rollback removes the column and the new index only.
+	require.NoError(t, migrations.MigrateToVersion(ctx, pool, migrationsPath, 108))
+	var colExists, newIdx, oldIdx bool
+	require.NoError(t, pool.QueryRow(ctx, `SELECT EXISTS (SELECT 1 FROM information_schema.columns WHERE table_name = 'recommendations' AND column_name = 'availability_zone')`).Scan(&colExists))
+	require.NoError(t, pool.QueryRow(ctx, `SELECT EXISTS (SELECT 1 FROM pg_indexes WHERE indexname = 'recommendations_natural_key_az_idx')`).Scan(&newIdx))
+	require.NoError(t, pool.QueryRow(ctx, `SELECT EXISTS (SELECT 1 FROM pg_indexes WHERE indexname = 'recommendations_natural_key_idx')`).Scan(&oldIdx))
+	assert.False(t, colExists)
+	assert.False(t, newIdx)
+	assert.True(t, oldIdx)
+}