Shortcut concurrent index creation/removal if no effect.

Index creation does not have an effect if the index is present already.
Index removal does not have an affect if the index is not present.

This helps to avoid patterns like this in migrations:
```
if index_exists?(...)
  remove_concurrent_index(...)
end
```

1 files changed, 52 insertions, 10 deletions

diff --git a/spec/lib/gitlab/database/migration_helpers_spec.rb b/spec/lib/gitlab/database/migration_helpers_spec.rb
index 1de3a14b809..9074e17ae80 100644
--- a/spec/lib/gitlab/database/migration_helpers_spec.rb
+++ b/spec/lib/gitlab/database/migration_helpers_spec.rb
@@ -67,17 +67,35 @@ describe Gitlab::Database::MigrationHelpers do
 
           model.add_concurrent_index(:users, :foo, unique: true)
         end
+
+        it 'does nothing if the index exists already' do
+          expect(model).to receive(:index_exists?)
+            .with(:users, :foo, { algorithm: :concurrently, unique: true }).and_return(true)
+          expect(model).not_to receive(:add_index)
+
+          model.add_concurrent_index(:users, :foo, unique: true)
+        end
       end
 
       context 'using MySQL' do
-        it 'creates a regular index' do
-          expect(Gitlab::Database).to receive(:postgresql?).and_return(false)
+        before do
+          allow(Gitlab::Database).to receive(:postgresql?).and_return(false)
+        end
 
+        it 'creates a regular index' do
           expect(model).to receive(:add_index)
             .with(:users, :foo, {})
 
           model.add_concurrent_index(:users, :foo)
         end
+
+        it 'does nothing if the index exists already' do
+          expect(model).to receive(:index_exists?)
+            .with(:users, :foo, { unique: true }).and_return(true)
+          expect(model).not_to receive(:add_index)
+
+          model.add_concurrent_index(:users, :foo, unique: true)
+        end
       end
     end
 
@@ -95,6 +113,7 @@ describe Gitlab::Database::MigrationHelpers do
     context 'outside a transaction' do
       before do
         allow(model).to receive(:transaction_open?).and_return(false)
+        allow(model).to receive(:index_exists?).and_return(true)
       end
 
       context 'using PostgreSQL' do
@@ -103,18 +122,41 @@ describe Gitlab::Database::MigrationHelpers do
           allow(model).to receive(:disable_statement_timeout)
         end
 
-        it 'removes the index concurrently by column name' do
-          expect(model).to receive(:remove_index)
-            .with(:users, { algorithm: :concurrently, column: :foo })
+        describe 'by column name' do
+          it 'removes the index concurrently' do
+            expect(model).to receive(:remove_index)
+              .with(:users, { algorithm: :concurrently, column: :foo })
 
-          model.remove_concurrent_index(:users, :foo)
+            model.remove_concurrent_index(:users, :foo)
+          end
+
+          it 'does nothing if the index does not exist' do
+            expect(model).to receive(:index_exists?)
+              .with(:users, :foo, { algorithm: :concurrently, unique: true }).and_return(false)
+            expect(model).not_to receive(:remove_index)
+
+            model.remove_concurrent_index(:users, :foo, unique: true)
+          end
         end
 
-        it 'removes the index concurrently by index name' do
-          expect(model).to receive(:remove_index)
-            .with(:users, { algorithm: :concurrently, name: "index_x_by_y" })
+        describe 'by index name' do
+          before do
+            allow(model).to receive(:index_exists_by_name?).with(:users, "index_x_by_y").and_return(true)
+          end
+
+          it 'removes the index concurrently by index name' do
+            expect(model).to receive(:remove_index)
+              .with(:users, { algorithm: :concurrently, name: "index_x_by_y" })
+
+            model.remove_concurrent_index_by_name(:users, "index_x_by_y")
+          end
+
+          it 'does nothing if the index does not exist' do
+            expect(model).to receive(:index_exists_by_name?).with(:users, "index_x_by_y").and_return(false)
+            expect(model).not_to receive(:remove_index)
 
-          model.remove_concurrent_index_by_name(:users, "index_x_by_y")
+            model.remove_concurrent_index_by_name(:users, "index_x_by_y")
+          end
         end
       end