indexer minor: more efficient total tx query (MystenLabs#19004)

## Description 

title, per a related slack thread 

## Test plan 

CI

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API:

crates/sui-indexer/src/store/pg_indexer_store.rs

@@ -1224,9 +1224,9 @@ impl<T: R2D2Connection + 'static> PgIndexerStore<T> {
         read_only_blocking!(&self.blocking_cp, |conn| {
             checkpoints::table
                 .filter(checkpoints::epoch.eq(epoch as i64))
-                .select(max(checkpoints::network_total_transactions))
-                .first::<Option<i64>>(conn)
-                .map(|o| o.unwrap_or(0))
+                .select(checkpoints::network_total_transactions)
+                .order_by(checkpoints::sequence_number.desc())
+                .first::<i64>(conn)
         })
         .context("Failed to get network total transactions in epoch")
         .map(|v| v as u64)