Improve DB error message types

db/executor.go

@@ -165,7 +165,7 @@ func (e *executor) execute(ctx context.Context, ast *sql.Query) {
 			if err == nil {
 				res, err = e.operate(ctx, stm)
 			} else {
-				res, err = []interface{}{}, queryNotExecuted
+				res, err = []interface{}{}, errQueryNotExecuted
 			}
 
 			rsp = &Response{

db/fetch.go

@@ -279,11 +279,11 @@ func (e *executor) fetchPaths(ctx context.Context, doc *data.Doc, exprs ...sql.E
 		case sql.DOT:
 			return e.fetchPaths(ctx, doc, exprs...)
 		case sql.OEDGE:
-			return nil, featureNotImplemented
+			return nil, errFeatureNotImplemented
 		case sql.IEDGE:
-			return nil, featureNotImplemented
+			return nil, errFeatureNotImplemented
 		case sql.BEDGE:
-			return nil, featureNotImplemented
+			return nil, errFeatureNotImplemented
 		}
 	case *sql.PartExpression:
 		switch val := val.Part.(type) {

db/table.go

@@ -61,7 +61,7 @@ func (d *document) table(ctx context.Context, when method) (err error) {
 				// If there are GROUP BY clauses then
 				// let's calculate the
 
-				return featureNotImplemented
+				return errFeatureNotImplemented
 
 				ats := make([]interface{}, len(ft.Group))
 

db/vars.go

@@ -63,9 +63,31 @@ const (
 )
 
 var (
+	// workerCount specifies how many workers should be used
+	// to process each query statement concurrently.
 	workerCount = runtime.NumCPU() * 2
-	queryNotExecuted      = errors.New("Query not executed")
+
+	// queryIdentFailed occurs when a permission query asks
+	// for a field, meaning a document has to be fetched.
 	queryIdentFailed = errors.New("Found ident but no doc available")
-	featureNotImplemented = errors.New("Feature is not yet implemented")
+
+	// errQueryNotExecuted occurs when a transaction has
+	// failed, and the following queries are not executed.
+	errQueryNotExecuted = errors.New("Query not executed")
+
+	// errRaceCondition occurs when a record which is locked
+	// for editing, is updated from within a subquery.
+	errRaceCondition = errors.New("Failed to update the same document recursively")
+
+	// errRecursiveOverload occurs when too many subqueries
+	// are executed within one other, causing an endless loop.
+	errRecursiveOverload = errors.New("Infinite loop when running recursive subqueries")
+
+	// errFeatureNotImplemented occurs when a feature which
+	// has not yet been implemented, has been used in a query.
+	errFeatureNotImplemented = errors.New("Feature is not yet implemented")
+
+	// paramSearchKeys specifies the order in which context
+	// variables should be checked for any specified value.
 	paramSearchKeys = []string{ctxKeySpec, ctxKeySubs, ctxKeyVars, ctxKeyKeep}
 )