S3 Select API Support for CSV (#6127)

Add support for trivial where clause cases

vendor/github.com/xwb1989/sqlparser/impossible_query.go

@@ -0,0 +1,39 @@
+/*
+Copyright 2017 Google 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 agreedto 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 sqlparser
+
+// FormatImpossibleQuery creates an impossible query in a TrackedBuffer.
+// An impossible query is a modified version of a query where all selects have where clauses that are
+// impossible for mysql to resolve. This is used in the vtgate and vttablet:
+//
+// - In the vtgate it's used for joins: if the first query returns no result, then vtgate uses the impossible
+// query just to fetch field info from vttablet
+// - In the vttablet, it's just an optimization: the field info is fetched once form MySQL, cached and reused
+// for subsequent queries
+func FormatImpossibleQuery(buf *TrackedBuffer, node SQLNode) {
+	switch node := node.(type) {
+	case *Select:
+		buf.Myprintf("select %v from %v where 1 != 1", node.SelectExprs, node.From)
+		if node.GroupBy != nil {
+			node.GroupBy.Format(buf)
+		}
+	case *Union:
+		buf.Myprintf("%v %s %v", node.Left, node.Type, node.Right)
+	default:
+		node.Format(buf)
+	}
+}