perf: replace correlated subquery in doubts GET with batched reply-count fetch

jest.setup.ts

@@ -86,7 +86,7 @@ jest.mock("react-hotkeys-hook", () => ({
     useHotkeys: () => {}
 }));
 
-jest.mock("@/lib/ratelimit", () => {
+jest.mock("@/lib/ratelimit/ratelimit", () => {
     const createLimiter = () => ({
         limit: jest.fn().mockResolvedValue({
             success: true,

src/app/api/doubts/route.ts

@@ -162,11 +162,6 @@
       conditions.push(eq(doubtsTable.isSolved, "unsolved"));
     }
 
-    const replyCountSql =
-      sql<number>`coalesce((SELECT count(*) FROM ${repliesTable} WHERE ${repliesTable.doubtId} = ${doubtsTable.id}), 0)`.mapWith(
-        Number,
-      );
-
     const [totalCountRow] = await db
       .select({ count: count() })
       .from(doubtsTable)
@@ -176,7 +171,6 @@
     const query = db
       .select({
         ...getTableColumns(doubtsTable),
-        replyCount: replyCountSql,
       })
       .from(doubtsTable);
 
@@ -190,7 +184,10 @@
     if (sort === "popular") {
       orderByFields.push(desc(doubtsTable.likes));
     } else if (sort === "most-replied") {
-      orderByFields.push(desc(replyCountSql));
+      // Needed here only to sort correctly at the DB level before paging.
+      // Not selected as a column, so it costs nothing for any other sort mode.
+      const replyCountOrderSql = sql`coalesce((SELECT count(*) FROM ${repliesTable} WHERE ${repliesTable.doubtId} = ${doubtsTable.id}), 0)`;
+      orderByFields.push(desc(replyCountOrderSql));
     }
     orderByFields.push(desc(doubtsTable.createdAt));
 
@@ -200,6 +197,25 @@
       .limit(limit)
       .offset(offset);
 
+    // Batch-fetch reply counts for just the returned page (max `limit` ids)
+    // instead of running a correlated subquery per row in the SELECT.
+    if (doubts.length > 0) {
+      const replyCountRows = await db
+        .select({ doubtId: repliesTable.doubtId, count: count() })
+        .from(repliesTable)
+        .where(inArray(repliesTable.doubtId, doubts.map((d: any) => d.id)))
+        .groupBy(repliesTable.doubtId);
+
+      const replyCountMap = new Map(
+        replyCountRows.map((r: any) => [r.doubtId, Number(r.count)]),
+      );
+
+      doubts = doubts.map((doubt: any) => ({
+        ...doubt,
+        replyCount: replyCountMap.get(doubt.id) ?? 0,
+      }));
+    }
+
     if (email && doubts.length > 0) {
       const userLikes = await db
         .select({ doubtId: likesTable.doubtId })
@@ -238,7 +254,7 @@
         .innerJoin(tagsTable, eq(doubtTagsTable.tagId, tagsTable.id))
         .where(inArray(doubtTagsTable.doubtId, doubts.map((d: any) => d.id)));
 
       const tagsByDoubt = tagRows.reduce<
         Record<number, { id: number; name: string; normalizedName: string }[]>
       >((acc, row) => {
         acc[row.doubtId] = acc[row.doubtId] || [];
@@ -417,7 +433,7 @@
       for (const name of normalizedTags) {
         const match = existingTagsMap.get(name);
         if (match) {
           savedTags.push(match);
         } else {
           tagsToInsert.push({
             name: name.replace(/\b\w/g, (char) => char.toUpperCase()),