Ignore shards that don't have the selected measurement in them.

Fixes #2815 and fixes #2818.

cmd/influxd/run/server_test.go

@@ -466,9 +466,9 @@ func TestServer_Query_Multiple_Measurements(t *testing.T) {
 
 	test.addQueries([]*Query{
 		&Query{
-			name:    "tag without field should return error",
+			name:    "measurement in one shard but not another shouldn't panic server",
 			command: `SELECT host,value  FROM db0.rp0.cpu`,
-			exp:     `{"results":[{"error":"select statement must include at least one field or function call"}]}`,
+			exp:     `{"results":[{"series":[{"name":"cpu","tags":{"host":"server01"},"columns":["time","value"],"values":[["2000-01-01T00:00:00Z",100]]}]}]}`,
 		},
 	}...)
 

tsdb/tx.go

@@ -169,13 +169,20 @@ func (tx *tx) CreateMapReduceJobs(stmt *influxql.SelectStatement, tagKeys []stri
 					continue
 				}
 
+				// get the codec for this measuremnt. If this is nil it just means this measurement was
+				// never written into this shard, so we can skip it and continue.
+				codec := shard.FieldCodec(m.Name)
+				if codec == nil {
+					continue
+				}
+
 				var mapper influxql.Mapper
 
 				mapper = &LocalMapper{
 					seriesKeys:   t.SeriesKeys,
 					db:           shard.DB(),
 					job:          job,
-					decoder:      shard.FieldCodec(m.Name),
+					decoder:      codec,
 					filters:      t.Filters,
 					whereFields:  whereFields,
 					selectFields: selectFields,