sql: use FastIntMap for QueryState.RangesPerNode

This commit switches `QueryState.RangesPerNode` from a `map[roachpb.NodeID]int`
to a `util.FastIntMap`. This avoids a pair of heap allocation as long as node
IDs say below 32 and the number of ranges for each node stays below 14.

```
name                   old time/op    new time/op    delta
KV/Scan/SQL/rows=1-10    94.5µs ± 4%    94.1µs ± 6%    ~     (p=0.796 n=10+10)

name                   old alloc/op   new alloc/op   delta
KV/Scan/SQL/rows=1-10    20.1kB ± 0%    19.9kB ± 0%  -0.84%  (p=0.000 n=10+9)

name                   old allocs/op  new allocs/op  delta
KV/Scan/SQL/rows=1-10       245 ± 0%       243 ± 0%  -0.82%  (p=0.000 n=10+9)
```

pkg/sql/physicalplan/replicaoracle/BUILD.bazel

@@ -12,6 +12,7 @@ go_library(
         "//pkg/rpc",
         "//pkg/settings/cluster",
         "//pkg/sql/sqlerrors",
+        "//pkg/util",
         "@com_github_cockroachdb_errors//:errors",
     ],
 )

pkg/sql/physicalplan/replicaoracle/oracle.go

@@ -23,6 +23,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/rpc"
 	"github.com/cockroachdb/cockroach/pkg/settings/cluster"
 	"github.com/cockroachdb/cockroach/pkg/sql/sqlerrors"
+	"github.com/cockroachdb/cockroach/pkg/util"
 	"github.com/cockroachdb/errors"
 )
 
@@ -106,14 +107,13 @@ var oracleFactories = map[Policy]OracleFactory{}
 // QueryState encapsulates the history of assignments of ranges to nodes
 // done by an oracle on behalf of one particular query.
 type QueryState struct {
-	RangesPerNode  map[roachpb.NodeID]int
+	RangesPerNode  util.FastIntMap
 	AssignedRanges map[roachpb.RangeID]roachpb.ReplicaDescriptor
 }
 
 // MakeQueryState creates an initialized QueryState.
 func MakeQueryState() QueryState {
 	return QueryState{
-		RangesPerNode:  make(map[roachpb.NodeID]int),
 		AssignedRanges: make(map[roachpb.RangeID]roachpb.ReplicaDescriptor),
 	}
 }
@@ -233,7 +233,7 @@ func (o *binPackingOracle) ChoosePreferredReplica(
 	minLoad := int(math.MaxInt32)
 	var leastLoadedIdx int
 	for i, repl := range replicas {
-		assignedRanges := queryState.RangesPerNode[repl.NodeID]
+		assignedRanges := queryState.RangesPerNode.GetDefault(int(repl.NodeID))
 		if assignedRanges != 0 && assignedRanges < o.maxPreferredRangesPerLeaseHolder {
 			return repl.ReplicaDescriptor, nil
 		}

pkg/sql/physicalplan/span_resolver.go

@@ -267,7 +267,8 @@ func (it *spanResolverIterator) ReplicaInfo(
 	if err != nil {
 		return roachpb.ReplicaDescriptor{}, err
 	}
-	it.queryState.RangesPerNode[repl.NodeID]++
+	prev := it.queryState.RangesPerNode.GetDefault(int(repl.NodeID))
+	it.queryState.RangesPerNode.Set(int(repl.NodeID), prev+1)
 	it.queryState.AssignedRanges[rngID] = repl
 	return repl, nil
 }