diff --git a/.changeset/lazy-pugs-smoke.md b/.changeset/lazy-pugs-smoke.md
new file mode 100644
index 0000000000..0fd3d30296
--- /dev/null
+++ b/.changeset/lazy-pugs-smoke.md
@@ -0,0 +1,15 @@
+---
+'@hyperdx/common-utils': patch
+'@hyperdx/app': patch
+---
+
+fix: disable per-part subcolumn size calculation on ClickHouse 26.3+
+
+ClickHouse 26.3 turned on
+`allow_calculating_subcolumns_sizes_for_merge_tree_reading` by default, which
+makes PREWHERE planning fetch per-part sizes for every map key a query
+references. On SharedMergeTree that is one S3 GET per (key × active part), it
+runs before any row is read, and `max_execution_time` does not interrupt it.
+Queries referencing many attribute keys — the LLM dashboard reads ~64 — could
+spend minutes in planning. Queries now send the setting as `0` when the server
+supports it.
diff --git a/packages/app/src/llm/dashboard/EfficiencyCharts.tsx b/packages/app/src/llm/dashboard/EfficiencyCharts.tsx
index 5a6fad867c..c3bf51c547 100644
--- a/packages/app/src/llm/dashboard/EfficiencyCharts.tsx
+++ b/packages/app/src/llm/dashboard/EfficiencyCharts.tsx
@@ -23,7 +23,8 @@ const PERCENT_NUMBER_FORMAT: NumberFormat = {
  */
 export function EfficiencyCharts(props: LLMChartProps) {
   const { source, expressions } = props;
-  const base = baseLLMChartConfig(props);
+  // Neither chart selects cost, so skip the ~70 KiB cost alias binding.
+  const base = baseLLMChartConfig({ ...props, withCostAlias: false });
 
   return (
     <>
diff --git a/packages/app/src/llm/dashboard/LatencyCharts.tsx b/packages/app/src/llm/dashboard/LatencyCharts.tsx
index 7a79509d77..07737db5c6 100644
--- a/packages/app/src/llm/dashboard/LatencyCharts.tsx
+++ b/packages/app/src/llm/dashboard/LatencyCharts.tsx
@@ -17,7 +17,8 @@ const CHART_HEIGHT = 320;
  */
 export function LatencyCharts(props: LLMChartProps) {
   const { source, expressions } = props;
-  const base = baseLLMChartConfig(props);
+  // Neither chart selects cost, so skip the ~70 KiB cost alias binding.
+  const base = baseLLMChartConfig({ ...props, withCostAlias: false });
 
   return (
     <>
diff --git a/packages/common-utils/src/__tests__/clickhouse.test.ts b/packages/common-utils/src/__tests__/clickhouse.test.ts
index 14e13606bd..a9b4825660 100644
--- a/packages/common-utils/src/__tests__/clickhouse.test.ts
+++ b/packages/common-utils/src/__tests__/clickhouse.test.ts
@@ -414,6 +414,10 @@ describe('processClickhouseSettings - optimization settings', () => {
       // { name: 'use_top_k_dynamic_filtering', value: '1' },
       { name: 'use_skip_indexes_on_data_read', value: '1' },
       { name: 'use_skip_indexes_for_disjunctions', value: '1' },
+      {
+        name: 'allow_calculating_subcolumns_sizes_for_merge_tree_reading',
+        value: '1',
+      },
     ]);
 
     await client.query({
@@ -440,9 +444,13 @@ describe('processClickhouseSettings - optimization settings', () => {
       // use_top_k_dynamic_filtering: '1',
       use_skip_indexes_on_data_read: '1',
       use_skip_indexes_for_disjunctions: '1',
+      allow_calculating_subcolumns_sizes_for_merge_tree_reading: '0',
     });
   });
 
+  // The exact-equality assertion below is what keeps unavailable settings off
+  // the wire — e.g. a pre-26.3 server that would reject
+  // allow_calculating_subcolumns_sizes_for_merge_tree_reading as unknown.
   it('should only apply available optimization settings', async () => {
     setupMockQuery([
       { name: 'use_skip_indexes_for_top_k', value: '1' },
diff --git a/packages/common-utils/src/clickhouse/index.ts b/packages/common-utils/src/clickhouse/index.ts
index 9df6177583..6a18dc1a2e 100644
--- a/packages/common-utils/src/clickhouse/index.ts
+++ b/packages/common-utils/src/clickhouse/index.ts
@@ -546,6 +546,15 @@ export abstract class BaseClickhouseClient {
     // Enables full-text (inverted index) search.
     applySettingIfAvailable('enable_full_text_index', '1');
 
+    // 26.3 turned this on by default. On SharedMergeTree it makes PREWHERE
+    // planning fetch per-part sizes for every map key referenced — one S3 GET
+    // each, not interruptible by max_execution_time. The sizes only reorder
+    // PREWHERE conditions, so the pre-26.3 approximation is fine.
+    applySettingIfAvailable(
+      'allow_calculating_subcolumns_sizes_for_merge_tree_reading',
+      '0',
+    );
+
     return {
       ...defaultSettings,
       ...clickhouse_settings,