From da6c505f41c91d7c1e97a53178a1b7412e9b89b4 Mon Sep 17 00:00:00 2001
From: Russ Egan <russ@safemonk.com>
Date: Fri, 6 Jan 2017 15:52:54 -0500
Subject: [PATCH] Avoid DDL when checking for versions table #134

---
 driver/postgres/postgres.go      | 8 ++++++++
 driver/postgres/postgres_test.go | 5 +++++
 2 files changed, 13 insertions(+)

diff --git a/driver/postgres/postgres.go b/driver/postgres/postgres.go
index 502c7d1..951b005 100644
--- a/driver/postgres/postgres.go
+++ b/driver/postgres/postgres.go
@@ -43,6 +43,14 @@ func (driver *Driver) Close() error {
 }
 
 func (driver *Driver) ensureVersionTableExists() error {
+	r := driver.db.QueryRow("SELECT count(*) FROM information_schema.tables WHERE table_name = $1;", tableName)
+	c := 0
+	if err := r.Scan(&c); err != nil {
+		return err
+	}
+	if c > 0 {
+		return nil
+	}
 	if _, err := driver.db.Exec("CREATE TABLE IF NOT EXISTS " + tableName + " (version int not null primary key);"); err != nil {
 		return err
 	}
diff --git a/driver/postgres/postgres_test.go b/driver/postgres/postgres_test.go
index a1c0495..7c5c0c3 100644
--- a/driver/postgres/postgres_test.go
+++ b/driver/postgres/postgres_test.go
@@ -37,6 +37,11 @@ func TestMigrate(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// testing idempotency: second call should be a no-op, since table already exists
+	if err := d.Initialize(driverUrl); err != nil {
+		t.Fatal(err)
+	}
+
 	files := []file.File{
 		{
 			Path:      "/foobar",