seaweedfs/weed/query/engine/engine.go

package engine

import (
	"context"
	"fmt"
	"strings"

	"github.com/seaweedfs/seaweedfs/weed/query/sqltypes"
	"github.com/xwb1989/sqlparser"
)

// SQLEngine provides SQL query execution capabilities for SeaweedFS
// Assumptions:
// 1. MQ namespaces map directly to SQL databases
// 2. MQ topics map directly to SQL tables  
// 3. Schema evolution is handled transparently with backward compatibility
// 4. Queries run against Parquet-stored MQ messages
type SQLEngine struct {
	catalog *SchemaCatalog
}

// QueryResult represents the result of a SQL query execution
type QueryResult struct {
	Columns []string                 `json:"columns"`
	Rows    [][]sqltypes.Value       `json:"rows"`
	Error   error                    `json:"error,omitempty"`
}

// NewSQLEngine creates a new SQL execution engine
// Assumption: Schema catalog is initialized with current MQ state
func NewSQLEngine() *SQLEngine {
	return &SQLEngine{
		catalog: NewSchemaCatalog(),
	}
}

// ExecuteSQL parses and executes a SQL statement
// Assumptions:
// 1. All SQL statements are MySQL-compatible via xwb1989/sqlparser
// 2. DDL operations (CREATE/ALTER/DROP) modify underlying MQ topics
// 3. DML operations (SELECT) query Parquet files directly
// 4. Error handling follows MySQL conventions
func (e *SQLEngine) ExecuteSQL(ctx context.Context, sql string) (*QueryResult, error) {
	// Parse the SQL statement
	stmt, err := sqlparser.Parse(sql)
	if err != nil {
		return &QueryResult{
			Error: fmt.Errorf("SQL parse error: %v", err),
		}, err
	}

	// Route to appropriate handler based on statement type
	switch stmt := stmt.(type) {
	case *sqlparser.Show:
		return e.executeShowStatement(ctx, stmt)
	case *sqlparser.DDL:
		return e.executeDDLStatement(ctx, stmt)
	case *sqlparser.Select:
		return e.executeSelectStatement(ctx, stmt)
	default:
		err := fmt.Errorf("unsupported SQL statement type: %T", stmt)
		return &QueryResult{Error: err}, err
	}
}

// executeShowStatement handles SHOW commands (DATABASES, TABLES, etc.)
// Assumption: These map directly to MQ namespace/topic metadata
func (e *SQLEngine) executeShowStatement(ctx context.Context, stmt *sqlparser.Show) (*QueryResult, error) {
	switch strings.ToUpper(stmt.Type) {
	case "DATABASES":
		return e.showDatabases(ctx)
	case "TABLES":
		// TODO: Parse FROM clause properly for database specification
		return e.showTables(ctx, "")
	default:
		err := fmt.Errorf("unsupported SHOW statement: %s", stmt.Type)
		return &QueryResult{Error: err}, err
	}
}

// executeDDLStatement handles CREATE, ALTER, DROP operations
// Assumption: These operations modify the underlying MQ topic structure
func (e *SQLEngine) executeDDLStatement(ctx context.Context, stmt *sqlparser.DDL) (*QueryResult, error) {
	switch stmt.Action {
	case sqlparser.CreateStr:
		return e.createTable(ctx, stmt)
	case sqlparser.AlterStr:
		return e.alterTable(ctx, stmt)
	case sqlparser.DropStr:
		return e.dropTable(ctx, stmt)
	default:
		err := fmt.Errorf("unsupported DDL action: %s", stmt.Action)
		return &QueryResult{Error: err}, err
	}
}

// executeSelectStatement handles SELECT queries
// Assumptions:
// 1. Queries run against Parquet files in MQ topics
// 2. Predicate pushdown is used for efficiency
// 3. Cross-topic joins are supported via partition-aware execution
func (e *SQLEngine) executeSelectStatement(ctx context.Context, stmt *sqlparser.Select) (*QueryResult, error) {
	// TODO: Implement SELECT query execution
	// This will involve:
	// 1. Query planning and optimization
	// 2. Parquet file scanning with predicate pushdown  
	// 3. Result set construction
	// 4. Streaming for large results
	
	err := fmt.Errorf("SELECT statement execution not yet implemented")
	return &QueryResult{Error: err}, err
}

// Helper methods for specific operations

func (e *SQLEngine) showDatabases(ctx context.Context) (*QueryResult, error) {
	databases := e.catalog.ListDatabases()
	
	result := &QueryResult{
		Columns: []string{"Database"},
		Rows:    make([][]sqltypes.Value, len(databases)),
	}
	
	for i, db := range databases {
		result.Rows[i] = []sqltypes.Value{
			sqltypes.NewVarChar(db),
		}
	}
	
	return result, nil
}

func (e *SQLEngine) showTables(ctx context.Context, dbName string) (*QueryResult, error) {
	// Assumption: If no database specified, use default or return error
	if dbName == "" {
		// TODO: Implement default database context
		// For now, use 'default' as the default database
		dbName = "default"
	}
	
	tables, err := e.catalog.ListTables(dbName)
	if err != nil {
		return &QueryResult{Error: err}, err
	}
	
	result := &QueryResult{
		Columns: []string{"Tables_in_" + dbName},
		Rows:    make([][]sqltypes.Value, len(tables)),
	}
	
	for i, table := range tables {
		result.Rows[i] = []sqltypes.Value{
			sqltypes.NewVarChar(table),
		}
	}
	
	return result, nil
}

func (e *SQLEngine) createTable(ctx context.Context, stmt *sqlparser.DDL) (*QueryResult, error) {
	// TODO: Implement table creation
	// This will create a new MQ topic with the specified schema
	err := fmt.Errorf("CREATE TABLE not yet implemented")
	return &QueryResult{Error: err}, err
}

func (e *SQLEngine) alterTable(ctx context.Context, stmt *sqlparser.DDL) (*QueryResult, error) {
	// TODO: Implement table alteration
	// This will modify the MQ topic schema with versioning
	err := fmt.Errorf("ALTER TABLE not yet implemented")
	return &QueryResult{Error: err}, err
}

func (e *SQLEngine) dropTable(ctx context.Context, stmt *sqlparser.DDL) (*QueryResult, error) {
	// TODO: Implement table dropping
	// This will delete the MQ topic
	err := fmt.Errorf("DROP TABLE not yet implemented")
	return &QueryResult{Error: err}, err
}