diff --git a/aggregator_core/src/datastore.rs b/aggregator_core/src/datastore.rs
index 9197fd5da..545f3a83c 100644
--- a/aggregator_core/src/datastore.rs
+++ b/aggregator_core/src/datastore.rs
@@ -1191,14 +1191,23 @@ impl<C: Clock> Transaction<'_, C> {
         A::InputShare: PartialEq,
         A::PublicShare: PartialEq,
     {
+        // Fetch the task's primary key and calculate the report timestamp threshold in a separate
+        // query. This will allow the query planner to make more accurate row count estimates, by
+        // comparing concrete values to the statistics of the `client_reports` table, and help the
+        // query planner reason that there will be only one task_id, so it can satisfy the `ORDER BY
+        // client_timestamp DESC` clause by using a reverse index scan, without an intermediate
+        // sort.
+        let (id, threshold) = self
+            .get_task_primary_key_and_expiry_threshold(task_id)
+            .await?;
+
         let stmt = self
             .prepare_cached(
                 "WITH unaggregated_reports AS (
                     SELECT client_reports.id FROM client_reports
-                    JOIN tasks ON tasks.id = client_reports.task_id
-                    WHERE tasks.task_id = $1
+                    WHERE client_reports.task_id = $1
                       AND client_reports.aggregation_started = FALSE
-                      AND client_reports.client_timestamp >= COALESCE($2::TIMESTAMP - tasks.report_expiry_age * '1 second'::INTERVAL, '-infinity'::TIMESTAMP)
+                      AND client_reports.client_timestamp >= $2
                     ORDER BY client_timestamp DESC
                     FOR UPDATE OF client_reports SKIP LOCKED
                     LIMIT $5::BIGINT
@@ -1213,8 +1222,8 @@ impl<C: Clock> Transaction<'_, C> {
             .query(
                 &stmt,
                 &[
-                    /* task_id */ &task_id.as_ref(),
-                    /* now */ &self.clock.now().as_naive_date_time()?,
+                    /* task_id */ &id,
+                    /* threshold */ &threshold,
                     /* updated_at */ &self.clock.now().as_naive_date_time()?,
                     /* updated_by */ &self.name,
                     /* limit */ &i64::try_from(limit)?,
@@ -4323,30 +4332,11 @@ impl<C: Clock> Transaction<'_, C> {
         // client_timestamp column's histogram, and make an accurate row count estimate. If the
         // threshold is determined in a single query via a join, the query planner is not able to
         // predict the task's report_expiry_age, and the accuracy of the row count estimate suffers.
-        let stmt_1 = self
-            .prepare_cached(
-                "SELECT
-                    id,
-                    COALESCE(
-                        $2::TIMESTAMP - tasks.report_expiry_age * '1 second'::INTERVAL,
-                        '-infinity'::TIMESTAMP
-                    ) AS threshold
-                FROM tasks WHERE tasks.task_id = $1",
-            )
+        let (id, threshold) = self
+            .get_task_primary_key_and_expiry_threshold(task_id)
             .await?;
-        let row = self
-            .query_one(
-                &stmt_1,
-                &[
-                    /* task_id */ &task_id.get_encoded(),
-                    /* now */ &self.clock.now().as_naive_date_time()?,
-                ],
-            )
-            .await?;
-        let id = row.get::<_, i64>("id");
-        let threshold = row.get::<_, Timestamp<NaiveDateTime>>("threshold");
 
-        let stmt_2 = self
+        let stmt = self
             .prepare_cached(
                 "WITH client_reports_to_delete AS (
                     SELECT client_reports.id FROM client_reports
@@ -4360,7 +4350,7 @@ impl<C: Clock> Transaction<'_, C> {
             )
             .await?;
         self.execute(
-            &stmt_2,
+            &stmt,
             &[
                 /* id */ &id,
                 /* threshold */ &threshold,
@@ -4371,6 +4361,37 @@ impl<C: Clock> Transaction<'_, C> {
         .map_err(Into::into)
     }
 
+    /// Helper function to look up a task's primary key, and compute a garbage collection visibility
+    /// threshold timestamp from its report expiry duration.
+    async fn get_task_primary_key_and_expiry_threshold(
+        &self,
+        task_id: &TaskId,
+    ) -> Result<(i64, Timestamp<NaiveDateTime>), Error> {
+        let stmt = self
+            .prepare_cached(
+                "SELECT
+                id,
+                COALESCE(
+                    $2::TIMESTAMP - tasks.report_expiry_age * '1 second'::INTERVAL,
+                    '-infinity'::TIMESTAMP
+                ) AS threshold
+            FROM tasks WHERE tasks.task_id = $1",
+            )
+            .await?;
+        let row = self
+            .query_one(
+                &stmt,
+                &[
+                    /* task_id */ &task_id.get_encoded(),
+                    /* now */ &self.clock.now().as_naive_date_time()?,
+                ],
+            )
+            .await?;
+        let id = row.get::<_, i64>("id");
+        let threshold = row.get::<_, Timestamp<NaiveDateTime>>("threshold");
+        Ok((id, threshold))
+    }
+
     /// Deletes old aggregation artifacts (aggregation jobs/report aggregations) for a given task,
     /// that is, aggregation artifacts for which the aggregation job's maximum client timestamp is
     /// older than the task's report expiry age. Up to `limit` aggregation jobs will be deleted,