feat(vet): Add default query parameters for explain queries (#2543)

* feat(vet): add default query parameters for vet explain with MySQL
* feat(vet): Add default parameter values for PostgreSQL

Before this change, all parameters to a query would be set to NULL when explaining the query. For both PostgreSQL and MySQL, this caused the explain output to either error or return unhelpful results.

For MySQL, these NULL parameters would cause "Impossible WHERE clause" errors. For PostgreSQL, the EXPLAIN output would return plans without indexes.

The default parameters aren't complete and won't cover all cases. We're working on a fallback mechanism that would allow you to specify the explain parameters values directly. Stay tuned!

---------

Co-authored-by: Andrew Benton <[email protected]>

Author: kyleconroy

docs/howto/vet.md

@@ -89,6 +89,8 @@ rules:
 
 ### Rules using `EXPLAIN ...` output
 
+*Added in v1.20.0*
+
 The CEL expression environment has two variables containing `EXPLAIN ...` output,
 `postgresql.explain` and `mysql.explain`. `sqlc` only populates the variable associated with
 your configured database engine, and only when you have a
@@ -100,15 +102,15 @@ For the `postgresql` engine, `sqlc` runs
 EXPLAIN (ANALYZE false, VERBOSE, COSTS, SETTINGS, BUFFERS, FORMAT JSON) ...
 ```
 
-where `"..."` is your query string, and parses the output into a `PostgreSQLExplain` proto message.
+where `"..."` is your query string, and parses the output into a [`PostgreSQLExplain`](https://buf.build/sqlc/sqlc/docs/v1.20.0:vet#vet.PostgreSQLExplain) proto message.
 
 For the `mysql` engine, `sqlc` runs
 
 ```sql
 EXPLAIN FORMAT=JSON ...
 ```
 
-where `"..."` is your query string, and parses the output into a `MySQLExplain` proto message.
+where `"..."` is your query string, and parses the output into a [`MySQLExplain`](https://buf.build/sqlc/sqlc/docs/v1.20.0:vet#vet.MySQLExplain) proto message.
 
 These proto message definitions are too long to include here, but you can find them in the `protos`
 directory within the `sqlc` source tree.

internal/cmd/vet.go

@@ -170,16 +170,145 @@ func (p *pgxConn) Prepare(ctx context.Context, name, query string) error {
 	return err
 }
 
+// Return a default value for a PostgreSQL column based on its type. Returns nil
+// if the type is unknown.
+func pgDefaultValue(col *plugin.Column) any {
+	if col == nil {
+		return nil
+	}
+	if col.Type == nil {
+		return nil
+	}
+	typname := strings.TrimPrefix(col.Type.Name, "pg_catalog.")
+	switch typname {
+	case "any", "void":
+		if col.IsArray {
+			return []any{}
+		} else {
+			return nil
+		}
+	case "anyarray":
+		return []any{}
+	case "bool", "boolean":
+		if col.IsArray {
+			return []bool{}
+		} else {
+			return false
+		}
+	case "double", "double precision", "real":
+		if col.IsArray {
+			return []float32{}
+		} else {
+			return 0.1
+		}
+	case "json", "jsonb":
+		if col.IsArray {
+			return []string{}
+		} else {
+			return "{}"
+		}
+	case "citext", "string", "text", "varchar":
+		if col.IsArray {
+			return []string{}
+		} else {
+			return ""
+		}
+	case "bigint", "bigserial", "integer", "int", "int2", "int4", "int8", "serial":
+		if col.IsArray {
+			return []int{}
+		} else {
+			return 1
+		}
+	case "date", "time", "timestamp", "timestamptz":
+		if col.IsArray {
+			return []time.Time{}
+		} else {
+			return time.Time{}
+		}
+	case "uuid":
+		if col.IsArray {
+			return []string{}
+		} else {
+			return "00000000-0000-0000-0000-000000000000"
+		}
+	case "numeric", "decimal":
+		if col.IsArray {
+			return []string{}
+		} else {
+			return "0.1"
+		}
+	case "inet":
+		if col.IsArray {
+			return []string{}
+		} else {
+			return "192.168.0.1/24"
+		}
+	case "cidr":
+		if col.IsArray {
+			return []string{}
+		} else {
+			return "192.168.1/24"
+		}
+	default:
+		return nil
+	}
+}
+
+// Return a default value for a MySQL column based on its type. Returns nil
+// if the type is unknown.
+func mysqlDefaultValue(col *plugin.Column) any {
+	if col == nil {
+		return nil
+	}
+	if col.Type == nil {
+		return nil
+	}
+	switch col.Type.Name {
+	case "any":
+		return nil
+	case "bool":
+		return false
+	case "int", "bigint", "mediumint", "smallint", "tinyint", "bit":
+		return 1
+	case "decimal": // "numeric", "dec", "fixed"
+		// No perfect choice here to avoid "Impossible WHERE" but I think
+		// 0.1 is decent. It works for all cases where `scale` > 0 which
+		// should be the majority. For more information refer to
+		// https://dev.mysql.com/doc/refman/8.1/en/fixed-point-types.html.
+		return 0.1
+	case "float", "double":
+		return 0.1
+	case "date":
+		return "0000-00-00"
+	case "datetime", "timestamp":
+		return "0000-00-00 00:00:00"
+	case "time":
+		return "00:00:00"
+	case "year":
+		return "0000"
+	case "char", "varchar", "binary", "varbinary", "tinyblob", "blob",
+		"mediumblob", "longblob", "tinytext", "text", "mediumtext", "longtext":
+		return ""
+	case "json":
+		return "{}"
+	default:
+		return nil
+	}
+}
+
 func (p *pgxConn) Explain(ctx context.Context, query string, args ...*plugin.Parameter) (*vetEngineOutput, error) {
 	eQuery := "EXPLAIN (ANALYZE false, VERBOSE, COSTS, SETTINGS, BUFFERS, FORMAT JSON) " + query
 	eArgs := make([]any, len(args))
+	for i, a := range args {
+		eArgs[i] = pgDefaultValue(a.Column)
+	}
 	row := p.c.QueryRow(ctx, eQuery, eArgs...)
 	var result []json.RawMessage
 	if err := row.Scan(&result); err != nil {
 		return nil, err
 	}
 	if debug.Debug.DumpExplain {
-		fmt.Println(eQuery)
+		fmt.Println(eQuery, "with args", eArgs)
 		fmt.Println(string(result[0]))
 	}
 	var explain vet.PostgreSQLExplain
@@ -210,13 +339,16 @@ type mysqlExplainer struct {
 func (me *mysqlExplainer) Explain(ctx context.Context, query string, args ...*plugin.Parameter) (*vetEngineOutput, error) {
 	eQuery := "EXPLAIN FORMAT=JSON " + query
 	eArgs := make([]any, len(args))
+	for i, a := range args {
+		eArgs[i] = mysqlDefaultValue(a.Column)
+	}
 	row := me.QueryRowContext(ctx, eQuery, eArgs...)
 	var result json.RawMessage
 	if err := row.Scan(&result); err != nil {
 		return nil, err
 	}
 	if debug.Debug.DumpExplain {
-		fmt.Println(eQuery)
+		fmt.Println(eQuery, "with args", eArgs)
 		fmt.Println(string(result))
 	}
 	var explain vet.MySQLExplain