fix(outputs.sql): Cache the table existence result properly

The old code will keep checking the table existence when the
table was created before, for example, after restarting telegraf.

plugins/outputs/sql/sql.go

@@ -232,10 +232,18 @@ func (p *SQL) generateInsert(tablename string, columns []string) string {
 }
 
 func (p *SQL) tableExists(tableName string) bool {
+    if p.tables[tableName] {
+        return true
+    }
+
 	stmt := strings.Replace(p.TableExistsTemplate, "{TABLE}", quoteIdent(tableName), -1)
 
 	_, err := p.db.Exec(stmt)
-	return err == nil
+	if err == nil {
+        p.tables[tableName] = true
+        return true
+    }
+    return false
 }
 
 func (p *SQL) Write(metrics []telegraf.Metric) error {
@@ -245,13 +253,12 @@ func (p *SQL) Write(metrics []telegraf.Metric) error {
 		tablename := metric.Name()
 
 		// create table if needed
-		if !p.tables[tablename] && !p.tableExists(tablename) {
+		if !p.tableExists(tablename) {
 			createStmt := p.generateCreateTable(metric)
 			_, err := p.db.Exec(createStmt)
 			if err != nil {
 				return err
 			}
-			p.tables[tablename] = true
 		}
 
 		var columns []string