feat(relations): 🎉 add flatten schema option for output

* Introduce `--flatten-schema` flag to convert, merge, and split commands.
* Modify database writing functions to support flattened schema names.
* Update template functions to handle schema.table naming convention.
* Enhance PostgreSQL writer to utilize flattened schema in generated SQL.
* Update tests to ensure compatibility with new flattening feature.
* Dependencies updated for improved functionality.

pkg/writers/pgsql/templates.go

@@ -18,14 +18,39 @@ type TemplateExecutor struct {
 	templates *template.Template
 }
 
-// NewTemplateExecutor creates a new template executor
-func NewTemplateExecutor() (*TemplateExecutor, error) {
+// NewTemplateExecutor creates a new template executor.
+// flattenSchema controls whether schema.table identifiers use dot or underscore separation.
+func NewTemplateExecutor(flattenSchema bool) (*TemplateExecutor, error) {
 	// Create template with custom functions
 	funcMap := make(template.FuncMap)
 	for k, v := range TemplateFunctions() {
 		funcMap[k] = v
 	}
 
+	// qual_table returns a quoted, schema-qualified identifier.
+	// With flatten=false: "schema"."table" (or unquoted equivalents).
+	// With flatten=true:  "schema_table".
+	funcMap["qual_table"] = func(schema, name string) string {
+		if schema == "" {
+			return quoteIdent(name)
+		}
+		if flattenSchema {
+			return quoteIdent(schema + "_" + name)
+		}
+		return quoteIdent(schema) + "." + quoteIdent(name)
+	}
+
+	// qual_table_raw is the same as qual_table but without identifier quoting.
+	funcMap["qual_table_raw"] = func(schema, name string) string {
+		if schema == "" {
+			return name
+		}
+		if flattenSchema {
+			return schema + "_" + name
+		}
+		return schema + "." + name
+	}
+
 	tmpl, err := template.New("").Funcs(funcMap).ParseFS(templateFS, "templates/*.tmpl")
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse templates: %w", err)