fix: field metaqueries take fast path if predicate is only on `_measu…

…rement` (influxdata#21962)

* fix: field metaqueries take fast path if predicate is only on `_measurement`

* chore: update CHANGELOG

* test: add test for fields with measurement predicate

CHANGELOG.md

@@ -70,6 +70,7 @@ This release adds an embedded SQLite database for storing metadata required by t
 1. [21925](https://github.com/influxdata/influxdb/pull/21925): Upgrade to golang-jwt 3.2.1.
 1. [21946](https://github.com/influxdata/influxdb/pull/21946): Prevent silently dropped writes when there are overlapping shards.
 1. [21950](https://github.com/influxdata/influxdb/pull/21950): Invalid requests to /api/v2 subroutes now return 404 instead of a list of links.
+1. [21962](https://github.com/influxdata/influxdb/pull/21962): Flux metaqueries for `_field` take fast path if `_measurement` is the only predicate.
 
 ## v2.0.7 [2021-06-04]
 

query/stdlib/influxdata/influxdb/show_fields_with_meas_pred_test.flux

@@ -0,0 +1,95 @@
+package influxdb_test
+
+
+import "testing"
+
+option now = () => 2030-01-01T00:00:00Z
+
+inData = "
+#datatype,string,long,dateTime:RFC3339,string,string,string,double
+#group,false,false,false,true,true,true,false
+#default,_result,,,,,,
+,result,table,_time,_measurement,host,_field,_value
+,,0,2018-05-22T19:53:26Z,system,host.local,load1,1.83
+,,0,2018-05-22T19:53:36Z,system,host.local,load1,1.72
+,,0,2018-05-22T19:53:46Z,system,host.local,load1,1.74
+,,0,2018-05-22T19:53:56Z,system,host.local,load1,1.63
+,,0,2018-05-22T19:54:06Z,system,host.local,load1,1.91
+,,0,2018-05-22T19:54:16Z,system,host.local,load1,1.84
+
+,,1,2018-05-22T19:53:26Z,sys,host.local,load3,1.98
+,,1,2018-05-22T19:53:36Z,sys,host.local,load3,1.97
+,,1,2018-05-22T19:53:46Z,sys,host.local,load3,1.97
+,,1,2018-05-22T19:53:56Z,sys,host.local,load3,1.96
+,,1,2018-05-22T19:54:06Z,sys,host.local,load3,1.98
+,,1,2018-05-22T19:54:16Z,sys,host.local,load3,1.97
+
+,,2,2018-05-22T19:53:26Z,system,host.local,load5,1.95
+,,2,2018-05-22T19:53:36Z,system,host.local,load5,1.92
+,,2,2018-05-22T19:53:46Z,system,host.local,load5,1.92
+,,2,2018-05-22T19:53:56Z,system,host.local,load5,1.89
+,,2,2018-05-22T19:54:06Z,system,host.local,load5,1.94
+,,2,2018-05-22T19:54:16Z,system,host.local,load5,1.93
+
+,,3,2018-05-22T19:53:26Z,swap,host.global,used_percent,82.98
+,,3,2018-05-22T19:53:36Z,swap,host.global,used_percent,82.59
+,,3,2018-05-22T19:53:46Z,swap,host.global,used_percent,82.59
+,,3,2018-05-22T19:53:56Z,swap,host.global,used_percent,82.59
+,,3,2018-05-22T19:54:06Z,swap,host.global,used_percent,82.59
+,,3,2018-05-22T19:54:16Z,swap,host.global,used_percent,82.64
+
+#datatype,string,long,dateTime:RFC3339,string,string,string,long
+#group,false,false,false,true,true,true,false
+#default,_result,,,,,,
+,result,table,_time,_measurement,host,_field,_value
+,,4,2018-05-22T19:53:26Z,sys,host.global,load7,183
+,,4,2018-05-22T19:53:36Z,sys,host.global,load7,172
+,,4,2018-05-22T19:53:46Z,sys,host.global,load7,174
+,,4,2018-05-22T19:53:56Z,sys,host.global,load7,163
+,,4,2018-05-22T19:54:06Z,sys,host.global,load7,191
+,,4,2018-05-22T19:54:16Z,sys,host.global,load7,184
+
+,,5,2018-05-22T19:53:26Z,sys,host.local,load8,198
+,,5,2018-05-22T19:53:36Z,sys,host.local,load8,197
+,,5,2018-05-22T19:53:46Z,sys,host.local,load8,197
+,,5,2018-05-22T19:53:56Z,sys,host.local,load8,196
+,,5,2018-05-22T19:54:06Z,sys,host.local,load8,198
+,,5,2018-05-22T19:54:16Z,sys,host.local,load8,197
+
+,,6,2018-05-22T19:53:26Z,sys,host.global,load9,195
+,,6,2018-05-22T19:53:36Z,sys,host.global,load9,192
+,,6,2018-05-22T19:53:46Z,sys,host.global,load9,192
+,,6,2018-05-22T19:53:56Z,sys,host.global,load9,189
+,,6,2018-05-22T19:54:06Z,sys,host.global,load9,194
+,,6,2018-05-22T19:54:16Z,sys,host.global,load9,193
+
+,,7,2018-05-22T19:53:26Z,swp,host.global,used_percent,8298
+,,7,2018-05-22T19:53:36Z,swp,host.global,used_percent,8259
+,,7,2018-05-22T19:53:46Z,swp,host.global,used_percent,8259
+,,7,2018-05-22T19:53:56Z,swp,host.global,used_percent,8259
+,,7,2018-05-22T19:54:06Z,swp,host.global,used_percent,8259
+,,7,2018-05-22T19:54:16Z,swp,host.global,used_percent,8264
+"
+
+testcase show_fields_with_meas_pred {
+    result = testing.loadStorage(csv: inData)
+        |> range(start: 2018-01-01T00:00:00Z, stop: 2019-01-01T00:00:00Z)
+        |> filter(fn: (r) => r._measurement == "sys")
+        |> keep(columns: ["_field"])
+        |> group()
+        |> distinct(column: "_field")
+        |> sort()
+
+    out_fields = "
+#datatype,string,long,string
+#group,false,false,false
+#default,0,,
+,result,table,_value
+,,0,load3
+,,0,load7
+,,0,load8
+,,0,load9
+"
+
+    testing.diff(got: result, want: testing.loadMem(csv: out_fields)) |> yield()
+}

v1/services/storage/predicate_influxql.go

@@ -123,7 +123,10 @@ func (v *hasAnyTagKeys) Visit(node influxql.Node) influxql.Visitor {
 	}
 
 	if n, ok := node.(*influxql.VarRef); ok {
-		if n.Val != fieldKey && n.Val != measurementKey && n.Val != "$" {
+		// The influxql expression will have had references to "_measurement"
+		// remapped to "_name" at this point by reads.NodeToExpr, so be sure to
+		// check for the appropriate value here using the measurementRemap map.
+		if n.Val != fieldKey && n.Val != measurementRemap[measurementKey] && n.Val != "$" {
 			v.found = true
 			return nil
 		}