feat(migration): add fallback for column type conversion failures

* implement renaming of old column and adding new column on conversion failure
* update templates and tests to support new behavior

pkg/writers/pgsql/templates/alter_column_type_with_check.tmpl

@@ -1,6 +1,7 @@
 DO $$
 DECLARE
   current_type text;
+  renamed_column text;
 BEGIN
   SELECT pg_catalog.format_type(a.atttypid, a.atttypmod)
     INTO current_type
@@ -15,8 +16,15 @@ BEGIN
 
   IF current_type IS NOT NULL
      AND current_type <> ALL(ARRAY[{{.EquivalentTypes}}]) THEN
-    ALTER TABLE {{qual_table .SchemaName .TableName}}
-      ALTER COLUMN {{quote_ident .ColumnName}} TYPE {{.NewType}}{{if .UsingExpr}} USING {{.UsingExpr}}{{end}};
+    BEGIN
+      ALTER TABLE {{qual_table .SchemaName .TableName}}
+        ALTER COLUMN {{quote_ident .ColumnName}} TYPE {{.NewType}}{{if .UsingExpr}} USING {{.UsingExpr}}{{end}};
+    EXCEPTION WHEN OTHERS THEN
+      renamed_column := '{{.ColumnName}}_' || trim(both '_' from regexp_replace(lower(current_type), '[^a-z0-9]+', '_', 'g'));
+      EXECUTE format('ALTER TABLE {{qual_table .SchemaName .TableName}} RENAME COLUMN {{quote_ident .ColumnName}} TO %I', renamed_column);
+      ALTER TABLE {{qual_table .SchemaName .TableName}}
+        ADD COLUMN {{quote_ident .ColumnName}} {{.NewType}};
+    END;
   END IF;
 END;
 $$;

pkg/writers/pgsql/templates/alter_column_type_with_fallback.tmpl

@@ -0,0 +1,13 @@
+DO $$
+BEGIN
+  BEGIN
+    ALTER TABLE {{qual_table .SchemaName .TableName}}
+      ALTER COLUMN {{quote_ident .ColumnName}} TYPE {{.NewType}}{{if .UsingExpr}} USING {{.UsingExpr}}{{end}};
+  EXCEPTION WHEN OTHERS THEN
+    ALTER TABLE {{qual_table .SchemaName .TableName}}
+      RENAME COLUMN {{quote_ident .ColumnName}} TO {{quote_ident .OldColumnName}};
+    ALTER TABLE {{qual_table .SchemaName .TableName}}
+      ADD COLUMN {{quote_ident .ColumnName}} {{.NewType}};
+  END;
+END;
+$$;