diff --git a/packages/realm-server/tests/matches-filter-integration-test.ts b/packages/realm-server/tests/matches-filter-integration-test.ts
index a0bb946efa9..ddd0a1c3ff4 100644
--- a/packages/realm-server/tests/matches-filter-integration-test.ts
+++ b/packages/realm-server/tests/matches-filter-integration-test.ts
@@ -357,33 +357,105 @@ module(basename(import.meta.filename), function () {
       );
     });
 
-    test('planner uses prerendered_html_markdown_fts_idx for matches queries', async function (assert) {
-      // The filter emits `to_tsvector('english', markdown_search_text(ph.markdown))`
-      // which must match the GIN index expression exactly. With only a handful
-      // of seeded rows PG will ordinarily prefer a seqscan; disabling seqscan
-      // forces the planner to reveal whether the GIN index is a viable
-      // candidate at all — which is the property we actually care about.
+    test('IndexQueryEngine emits the null-rejecting guard in its matches SQL', async function (assert) {
+      // The join-shape test below proves the guard *would* reduce the join, but
+      // it composes the predicate itself, so it can't catch IndexQueryEngine
+      // dropping the guard. This test closes that gap: it captures the SQL the
+      // engine actually emits for a `matches` search and asserts the guard is
+      // present, so a regression in the engine fails here rather than passing on
+      // a predicate the engine no longer produces.
+      let executed: string[] = [];
+      let originalExecute = dbAdapter.execute.bind(dbAdapter);
+      dbAdapter.execute = ((
+        sql: string,
+        opts?: Parameters<typeof originalExecute>[1],
+      ) => {
+        executed.push(sql);
+        return originalExecute(sql, opts);
+      }) as typeof dbAdapter.execute;
+      try {
+        await engine.searchCards(new URL(testRealmURL), {
+          filter: { matches: 'mango' },
+        });
+      } finally {
+        dbAdapter.execute = originalExecute;
+      }
+
+      let guard = `ph.markdown IS NOT NULL AND to_tsvector('english', markdown_search_text(ph.markdown))`;
+      let matchesSql = executed.filter((sql) =>
+        sql.includes('markdown_search_text(ph.markdown)'),
+      );
+      assert.ok(
+        matchesSql.length > 0,
+        'the engine ran a full-text matches query',
+      );
+      for (let sql of matchesSql) {
+        assert.ok(
+          sql.includes(guard),
+          `emitted matches SQL should include the null-rejecting guard; got: ${sql}`,
+        );
+      }
+    });
+
+    test('the null-rejecting guard reduces the prerendered_html LEFT JOIN to an inner join', async function (assert) {
+      // The engine attaches prerendered_html to boxel_index through a LEFT JOIN
+      // (see `prerenderedJoin`) and runs the `matches` predicate against the
+      // joined `ph`. `markdown_search_text` coalesces null markdown to '', which
+      // hides the predicate's null-rejection from the planner: without the
+      // `ph.markdown IS NOT NULL` guard the planner cannot reduce the outer join,
+      // so it drives the query from boxel_index and recomputes `to_tsvector` for
+      // every joined row instead of reaching `prerendered_html_markdown_fts_idx`.
+      // The guard restores the join reduction — and that reduction is exactly
+      // what makes the markdown GIN index reachable at scale (on a realm with
+      // thousands of rows the planner then BitmapAnds the realm B-tree with the
+      // markdown GIN; with only a handful of seeded rows here it prefers the
+      // realm B-tree plus a filter, so asserting the index name would be
+      // dataset-dependent). The join type is the size-independent signal, so we
+      // assert directly on it: the guarded predicate yields an inner join and the
+      // unguarded predicate stays a left join. A build that drops the guard fails
+      // here rather than silently reintroducing the slowdown.
       //
-      // SET LOCAL is bound to a transaction, so we run SET/EXPLAIN/COMMIT on
-      // the same pooled connection via withConnection. Each inner query is
-      // its own round-trip, so EXPLAIN's result is returned cleanly.
-      let plan = await dbAdapter.withConnection(async (run) => {
-        await run(['BEGIN']);
-        await run(['SET LOCAL enable_seqscan = OFF']);
-        let rows = await run([
-          `EXPLAIN (FORMAT JSON)
-           SELECT url FROM prerendered_html
-           WHERE to_tsvector('english', markdown_search_text(markdown))
-                 @@ websearch_to_tsquery('english', 'mango')`,
-        ]);
-        await run(['COMMIT']);
-        return rows;
-      });
+      // EXPLAIN's JSON output stringifies each join's planner-chosen strategy as
+      // `"Join Type":"Inner"` / `"Join Type":"Left"`; this query has exactly one
+      // join, so a substring check is unambiguous.
+      //
+      // The realm URL and query term are bound as parameters ($1/$2) rather than
+      // interpolated, so the SQL stays quoting-safe whatever their contents; only
+      // the predicate *shape* — which is test-controlled, not data — is composed
+      // inline.
+      let tsPredicate = `to_tsvector('english', markdown_search_text(ph.markdown)) @@ websearch_to_tsquery('english', $2)`;
+      let joinShapeExplain = (predicate: string) =>
+        `EXPLAIN (FORMAT JSON)
+         SELECT i.url
+         FROM boxel_index AS i
+         LEFT JOIN prerendered_html AS ph
+           ON ph.url = i.url AND ph.realm_url = i.realm_url AND ph.type = i.type
+         WHERE i.realm_url = $1
+           AND ${predicate}`;
+
+      let explainJoinShape = async (predicate: string) =>
+        JSON.stringify(
+          await dbAdapter.execute(joinShapeExplain(predicate), {
+            bind: [testRealmURL, 'mango'],
+          }),
+        );
+
+      let guarded = await explainJoinShape(
+        `ph.markdown IS NOT NULL AND ${tsPredicate}`,
+      );
+      let unguarded = await explainJoinShape(tsPredicate);
 
-      let planText = JSON.stringify(plan);
       assert.ok(
-        planText.includes('prerendered_html_markdown_fts_idx'),
-        `plan should reference the GIN index; got: ${planText}`,
+        guarded.includes('"Join Type":"Inner"'),
+        `guarded predicate should reduce the outer join to an inner join; got: ${guarded}`,
+      );
+      assert.notOk(
+        guarded.includes('"Join Type":"Left"'),
+        `guarded predicate should leave no unreduced outer join; got: ${guarded}`,
+      );
+      assert.ok(
+        unguarded.includes('"Join Type":"Left"'),
+        `unguarded predicate should leave the outer join unreduced — the bug the guard fixes; got: ${unguarded}`,
       );
     });
   });
diff --git a/packages/runtime-common/index-query-engine.ts b/packages/runtime-common/index-query-engine.ts
index 9795b2a478a..04a2e4b3a94 100644
--- a/packages/runtime-common/index-query-engine.ts
+++ b/packages/runtime-common/index-query-engine.ts
@@ -1293,7 +1293,18 @@ export class IndexQueryEngine {
         : [
             dbExpression({
               pg: [
-                `to_tsvector('english', markdown_search_text(ph.markdown)) @@ websearch_to_tsquery('english',`,
+                // The leading `ph.markdown IS NOT NULL` is a null-rejecting
+                // guard, not a behavior change: `markdown_search_text` coalesces
+                // null markdown to '' and a non-empty query never matches an
+                // empty tsvector, so absent-markdown rows are already excluded.
+                // But because that coalesce hides the null-rejection from the
+                // planner, without this guard it cannot reduce the LEFT JOIN to
+                // prerendered_html, drives the query from boxel_index, and
+                // recomputes every row's tsvector at query time instead of using
+                // the markdown GIN index. The explicit guard restores the
+                // reduction so the index is used, and stays correct under `not`
+                // and `any` (an absent row still can't match a positive term).
+                `ph.markdown IS NOT NULL AND to_tsvector('english', markdown_search_text(ph.markdown)) @@ websearch_to_tsquery('english',`,
                 param(filter.matches),
                 `)`,
               ],