perf(promql): push down last row for instant queries (#9034)

* perf(promql): push down last row for instant queries

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* test: guard instant last row correctness

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* test: update instant query explain results

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* fix: apply last row after source deduplication

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* fix: scope post-merge last row selection

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* test(perf): cover instant PromQL last row

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* fix(perf): sort generated SST rows before writing

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* test(promql): cover instant last row selection in sqlness

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* fix(promql): avoid last row hints for lossy timestamp casts

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* test(promql): preserve stale marker semantics across flushes

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* test(promql): decode dictionary labels in stale regression

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* test(promql): avoid reserved column name in stale fixture

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* test(promql): exercise LastRow hints and filtered results

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* refactor: keep after-merge mode in LastRow selector

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* fix: reject instant LastRow across residual filters

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* test: expect after-merge selector in instant vector guards

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* docs: explain instant LastRow filter eligibility

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* fix: restrict instant LastRow to safe selector nodes

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

* refactor: show LastRow merge mode directly in diagnostics

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* test: refresh LastRow display in explain expectations

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---------

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.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 = [