perf(api): read only the matched metrics' tags rows in PromQL label lookups

.changeset/label-lookup-metric-name.md

@@ -0,0 +1,7 @@
+---
+'@hyperdx/api': patch
+'@hyperdx/common-utils': patch
+---
+
+perf: PromQL label lookups with `match[]` read only the matched metrics' tags
+rows when every selector names its metric

packages/api/src/controllers/__tests__/timeseriesEngine.int.test.ts

@@ -202,6 +202,17 @@ describe('timeseriesEngine controller', () => {
         ).toEqual(['api', 'batch']);
       });
 
+      // A selector without a metric name can match any metric, so naming
+      // `old_metric` in another selector mustn't narrow the lookup to it.
+      it('keeps the series of a selector without a metric name', async () => {
+        expect(
+          await labelValues({
+            labelName: 'job',
+            match: ['{job="web"}', 'old_metric'],
+          }),
+        ).toEqual(['batch', 'web']);
+      });
+
       it('scopes metric names to the selector', async () => {
         expect(
           await labelValues({

packages/api/src/controllers/timeseriesEngine.ts

@@ -8,6 +8,7 @@ import {
   Metadata,
   MetadataCache,
 } from '@hyperdx/common-utils/dist/core/metadata';
+import { promqlSelectorMetricName } from '@hyperdx/common-utils/dist/core/promqlSelector';
 import {
   PrometheusMatrixResult,
   PrometheusVectorResult,
@@ -121,7 +122,16 @@ function getSeriesSelectorCondition({
       )`,
   );
 
-  return chSql`id IN (${concatChSql(' UNION DISTINCT ', idsPerSelector)})`;
+  const ids = chSql`id IN (${concatChSql(' UNION DISTINCT ', idsPerSelector)})`;
+  // The tags table is sorted by metric name, so `id IN (...)` alone reads every
+  // tags row. When each selector names its metric, say so: same rows, but only
+  // those metrics' part of the table is read.
+  const names = match
+    .map(promqlSelectorMetricName)
+    .filter((n): n is string => n != null);
+  if (names.length < match.length) return ids;
+  const list = [...new Set(names)].map(n => chSql`${{ String: n }}`);
+  return chSql`metric_name IN (${concatChSql(', ', list)}) AND ${ids}`;
 }
 
 /**

packages/common-utils/src/core/__tests__/promqlSelector.test.ts

@@ -0,0 +1,27 @@
+import { promqlSelectorMetricName } from '@/core/promqlSelector';
+
+describe('promqlSelectorMetricName', () => {
+  it.each([
+    ['up', 'up'],
+    [' node_cpu:rate5m{job="x"}', 'node_cpu:rate5m'],
+    ['{__name__="up"}', 'up'],
+    ['{job="x", __name__="up"}', 'up'],
+    ["{job=~'a|b', env!=`x`, __name__='up'}", 'up'],
+  ])('reads the metric of %s', (selector, name) => {
+    expect(promqlSelectorMetricName(selector)).toBe(name);
+  });
+
+  it.each([
+    '{job="x"}',
+    '{__name__=~"up|down"}',
+    '{job="x", __name__!="up"}',
+    '{job="__name__=\\"up\\""}',
+    `{job='a,__name__="up"'}`,
+    '{job=`a,__name__="up"`}',
+    '{__name__="a\\"b"}',
+    'sum(up)',
+    'up{',
+  ])('returns undefined for %s', selector => {
+    expect(promqlSelectorMetricName(selector)).toBeUndefined();
+  });
+});

packages/common-utils/src/core/promqlSelector.ts

@@ -0,0 +1,41 @@
+import { parser } from '@prometheus-io/lezer-promql';
+
+/**
+ * The metric a series selector is limited to: `name`, `name{...}`, or
+ * `{__name__="name", ...}`. Undefined when the selector may match more than
+ * one metric, isn't a single plain selector, or doesn't parse.
+ */
+export function promqlSelectorMetricName(selector: string): string | undefined {
+  const tree = parser.parse(selector);
+  let parseError = false;
+  tree.iterate({
+    enter: node => {
+      if (node.type.isError) parseError = true;
+    },
+  });
+  const vectorSelector = tree.topNode.firstChild;
+  if (parseError || vectorSelector?.name !== 'VectorSelector') return;
+
+  const identifier = vectorSelector.getChild('Identifier');
+  if (identifier) return selector.slice(identifier.from, identifier.to);
+
+  const matchers =
+    vectorSelector
+      .getChild('LabelMatchers')
+      ?.getChildren('UnquotedLabelMatcher') ?? [];
+  for (const matcher of matchers) {
+    const name = matcher.getChild('LabelName');
+    const op = matcher.getChild('MatchOp')?.firstChild;
+    const value = matcher.getChild('StringLiteral');
+    if (
+      name &&
+      value &&
+      op?.name === 'EqlSingle' &&
+      selector.slice(name.from, name.to) === '__name__'
+    ) {
+      // Values with escapes would need unquoting; fall back instead.
+      const text = selector.slice(value.from + 1, value.to - 1);
+      return text.includes('\\') ? undefined : text;
+    }
+  }
+}