diff --git a/.github/scripts/query-regression-run.py b/.github/scripts/query-regression-run.py
index d293aa3e0c9..268cb0bae6c 100644
--- a/.github/scripts/query-regression-run.py
+++ b/.github/scripts/query-regression-run.py
@@ -38,6 +38,7 @@ DEFAULT_CASES = [
     "tests/perf/query_cases/prom_remote_write_mixed_every/case.toml",
     "tests/perf/query_cases/prom_remote_write_integer_counter/case.toml",
     "tests/perf/query_cases/promql_range_boundary/case.toml",
+    "tests/perf/query_cases/promql_instant_last_row_9034/case.toml",
 ]
 
 HEAVY_CASES = [
diff --git a/tests/perf/README.md b/tests/perf/README.md
index 42fd8c286e0..d518fec996c 100644
--- a/tests/perf/README.md
+++ b/tests/perf/README.md
@@ -204,7 +204,7 @@ case for issue #7913. It writes 8192 series × 20160 samples through remote-writ
 in 1440-sample daily time chunks, flushing after each chunk before running 1d/7d/14d
 TQL selectors. It is not included in the default `all` case set because ingestion
 cost dominates routine CI validation. Commenting `/query-regression heavy` runs
-only this case; `/query-regression` runs the six routine default cases. Manual
+only this case; `/query-regression` runs the seven routine default cases. Manual
 workflow dispatch accepts the `heavy` token to select this case.
 
 ## OTLP trace load scenario
@@ -393,12 +393,12 @@ parquetbench/scanbench` as the read-bench tool against each target's data direct
 
 The workflow runs when an allowlisted repository admin comments
 `/query-regression` on a non-draft PR. It does not rerun on pushes,
-ready-for-review, or reopen events. `/query-regression` runs the six routine
-default cases; `/query-regression heavy` runs only the high-cardinality
-remote-write #7913 case. PR runs build base/candidate once and
-use `--allow-large-fixture`. Manual `workflow_dispatch` runs can pass `all`,
-`heavy`, one case path, or a comma/whitespace-separated list of case paths, and
-can override refs.
+ready-for-review, or reopen events. `/query-regression` runs the seven routine
+default cases, including `promql_instant_last_row_9034`;
+`/query-regression heavy` runs only the high-cardinality remote-write #7913 case.
+PR runs build base/candidate once and use `--allow-large-fixture`. Manual
+`workflow_dispatch` runs can pass `all`, `heavy`, one case path, or a
+comma/whitespace-separated list of case paths, and can override refs.
 
 Comment admission is two workflows. `slash-command-dispatch.yml` uses
 [peter-evans/slash-command-dispatch](https://github.com/peter-evans/slash-command-dispatch)
diff --git a/tests/perf/query_cases/promql_instant_last_row_9034/case.toml b/tests/perf/query_cases/promql_instant_last_row_9034/case.toml
new file mode 100644
index 00000000000..6dd5dc2fb8b
--- /dev/null
+++ b/tests/perf/query_cases/promql_instant_last_row_9034/case.toml
@@ -0,0 +1,92 @@
+# PromQL instant-selector latency regression coverage for #9034.
+#
+# Instant, range, and aggregate controls cover LastRow latency behavior. The
+# candidate instant query may enable the existing LastRow selector; the range
+# query must not. This is latency regression coverage only: optimizer and
+# sqlness tests own plan and correctness coverage. Evaluation is 0.7s after the
+# final generated sample (last timestamp = start + 1023 * 0.1s =
+# 1704067302.3) and remains inside the default lookback window.
+
+[case]
+name = "promql_instant_last_row_9034"
+description = "PromQL instant selector LastRow latency regression for #9034"
+
+[scenario]
+kind = "direct_readable_sst"
+seed = 9034
+
+[[scenario.tables]]
+database = "public"
+name = "promql_instant_last_row_9034"
+engine = "mito"
+append_mode = true
+sst_format = "flat"
+primary_key = ["host", "instance"]
+time_index = "ts"
+
+[[scenario.tables.columns]]
+name = "host"
+type = "STRING"
+semantic = "tag"
+distribution = { kind = "cardinality", values = 16, prefix = "host" }
+
+[[scenario.tables.columns]]
+name = "instance"
+type = "STRING"
+semantic = "tag"
+distribution = { kind = "cardinality", values = 256, prefix = "instance" }
+
+[[scenario.tables.columns]]
+name = "value"
+type = "DOUBLE"
+semantic = "field"
+distribution = { kind = "deterministic_wave", min = 0.0, max = 1000.0 }
+
+[[scenario.tables.columns]]
+name = "ts"
+type = "TIMESTAMP(9)"
+semantic = "timestamp"
+
+[scenario.layout]
+regions = 1
+sst_count = 64
+rows_per_sst = 4096
+row_group_size = 1024
+series_count = 256
+start_unix_nanos = 1704067200000000000
+step_nanos = 100000000
+time_range_layout = "non_overlapping_per_sst"
+series_layout = "timestamp_major"
+
+# Candidate instant query may enable the existing LastRow selector.
+[[scenario.queries]]
+name = "instant_selector_last_row_candidate"
+kind = "tql"
+query = "TQL ANALYZE VERBOSE (1704067303, 1704067303, '1s') promql_instant_last_row_9034{host=~'host.*'}"
+warmup = 3
+iterations = 9
+
+[scenario.queries.thresholds]
+max_candidate_latency_regression_pct = 25
+
+# True range control: this must not enable the LastRow selector.
+[[scenario.queries]]
+name = "range_selector_last_row_control"
+kind = "tql"
+query = "TQL ANALYZE VERBOSE (1704067293, 1704067303, '1s') promql_instant_last_row_9034{host=~'host.*'}"
+warmup = 3
+iterations = 9
+
+[scenario.queries.thresholds]
+max_candidate_latency_regression_pct = 25
+
+# This pre-existing aggregate-hint control should return one row per 256 series; it is latency regression coverage, not a result oracle.
+[[scenario.queries]]
+name = "sql_aggregate_last_row_control"
+kind = "sql"
+query = "SELECT host, instance, last_value(value ORDER BY ts ASC) FROM promql_instant_last_row_9034 GROUP BY host, instance"
+warmup = 3
+iterations = 9
+
+[scenario.queries.thresholds]
+max_candidate_latency_regression_pct = 25
diff --git a/tests/perf/test_query_regression_case_selection.py b/tests/perf/test_query_regression_case_selection.py
index 642b6ee1f2b..84b3cd44fa7 100644
--- a/tests/perf/test_query_regression_case_selection.py
+++ b/tests/perf/test_query_regression_case_selection.py
@@ -40,6 +40,7 @@ class QueryRegressionCaseSelectionTest(unittest.TestCase):
                 "tests/perf/query_cases/prom_remote_write_mixed_every/case.toml",
                 "tests/perf/query_cases/prom_remote_write_integer_counter/case.toml",
                 "tests/perf/query_cases/promql_range_boundary/case.toml",
+                "tests/perf/query_cases/promql_instant_last_row_9034/case.toml",
             ],
         )