Added the concept of an interpreter expression

engine.go

@@ -150,6 +150,8 @@ type Engine struct {
 	Parser            sql.Parser
 }
 
+var _ analyzer.StatementRunner = (*Engine)(nil)
+
 type ColumnWithRawDefault struct {
 	SqlColumn *sql.Column
 	Default   string
@@ -195,6 +197,7 @@ func New(a *analyzer.Analyzer, cfg *Config) *Engine {
 		Parser:            sql.GlobalParser,
 	}
 	ret.ReadOnly.Store(cfg.IsReadOnly)
+	a.Runner = ret
 	return ret
 }
 

enginetest/initialization.go

@@ -91,6 +91,7 @@ func NewEngineWithProvider(_ *testing.T, harness Harness, provider sql.DatabaseP
 	if idh, ok := harness.(IndexDriverHarness); ok {
 		idh.InitializeIndexDriver(engine.Analyzer.Catalog.AllDatabases(NewContext(harness)))
 	}
+	analyzer.Runner = engine
 
 	return engine
 }

sql/analyzer/analyzer.go

@@ -286,6 +286,8 @@ type Analyzer struct {
 	Coster memo.Coster
 	// ExecBuilder converts a sql.Node tree into an executable iterator.
 	ExecBuilder sql.NodeExecBuilder
+	// Runner represents the engine, which is represented as a separate interface to work around circular dependencies
+	Runner StatementRunner
 }
 
 // NewDefault creates a default Analyzer instance with all default Rules and configuration.

sql/analyzer/interpreter.go

@@ -0,0 +1,47 @@
+// Copyright 2022 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package analyzer
+
+import (
+	"github.com/dolthub/vitess/go/vt/sqlparser"
+
+	"github.com/dolthub/go-mysql-server/sql/transform"
+
+	"github.com/dolthub/go-mysql-server/sql"
+	"github.com/dolthub/go-mysql-server/sql/plan"
+)
+
+// Interpreter is an interface that implements an interpreter. These are typically used for functions (which may be
+// implemented as a set of operations that are interpreted during runtime).
+type Interpreter interface {
+	SetStatementRunner(ctx *sql.Context, runner StatementRunner) sql.Expression
+}
+
+// StatementRunner is essentially an interface that the engine will implement. We cannot directly reference the engine
+// here as it will cause an import cycle, so this may be updated to suit any function changes that the engine
+// experiences.
+type StatementRunner interface {
+	QueryWithBindings(ctx *sql.Context, query string, parsed sqlparser.Statement, bindings map[string]sqlparser.Expr, qFlags *sql.QueryFlags) (sql.Schema, sql.RowIter, *sql.QueryFlags, error)
+}
+
+// interpreter hands the engine to any interpreter expressions.
+func interpreter(ctx *sql.Context, a *Analyzer, n sql.Node, scope *plan.Scope, sel RuleSelector, qFlags *sql.QueryFlags) (sql.Node, transform.TreeIdentity, error) {
+	return transform.NodeExprs(n, func(expr sql.Expression) (sql.Expression, transform.TreeIdentity, error) {
+		if interp, ok := expr.(Interpreter); ok {
+			return interp.SetStatementRunner(ctx, a.Runner), transform.NewTree, nil
+		}
+		return expr, transform.SameTree, nil
+	})
+}

sql/analyzer/rule_ids.go

@@ -66,6 +66,7 @@ const (
 	assignRoutinesId             // assignRoutines
 	modifyUpdateExprsForJoinId   // modifyUpdateExprsForJoin
 	applyForeignKeysId           // applyForeignKeys
+	interpreterId                // interpreter
 
 	// validate
 	validateResolvedId          // validateResolved

sql/analyzer/rules.go

@@ -93,6 +93,7 @@ var OnceAfterDefault = []Rule{
 	{assignRoutinesId, assignRoutines},
 	{modifyUpdateExprsForJoinId, modifyUpdateExprsForJoin},
 	{applyForeignKeysId, applyForeignKeys},
+	{interpreterId, interpreter},
 }
 
 // DefaultValidationRules to apply while analyzing nodes.