disallow concurrently / add basic docs

Co-authored-by: Ryan Krage <[email protected]>

README.md

@@ -171,6 +171,14 @@ Unsafely make a column not nullable.
 unsafe_make_column_not_nullable :table, :column
 ```
 
+#### safe\_add\_index\_on\_empty\_table
+
+Safely add an index on a table with zero rows. This will raise an error if the table contains data.
+
+```ruby
+safe_add_index_on_empty_table :table, :column
+```
+
 #### safe\_add\_concurrent\_index
 
 Add an index concurrently.

lib/pg_ha_migrations/safe_statements.rb

@@ -155,6 +155,10 @@ def unsafe_make_column_not_nullable(table, column, options={}) # options arg is
   end
 
   def safe_add_index_on_empty_table(table, columns, options={})
+    if options[:algorithm] == :concurrently
+      raise ArgumentError, "Cannot call safe_add_index_on_empty_table with :algorithm => :concurrently"
+    end
+
     # Avoids taking out an unnecessary SHARE lock if the table does have data
     _ensure_empty_table!(table)
 

spec/safe_statements_spec.rb

@@ -1286,6 +1286,18 @@ def up
             )
           end
 
+          it "raises error when :algorithm => :concurrently provided" do
+            test_migration = Class.new(migration_klass) do
+              def up
+                safe_add_index_on_empty_table :foos, [:bar], algorithm: :concurrently
+              end
+            end
+
+            expect do
+              test_migration.suppress_messages { test_migration.migrate(:up) }
+            end.to raise_error(ArgumentError, "Cannot call safe_add_index_on_empty_table with :algorithm => :concurrently")
+          end
+
           it "raises error when table contains rows" do
             setup_migration = Class.new(migration_klass) do
               def up