chibenwa commented on code in PR #2514:
URL: https://github.com/apache/james-project/pull/2514#discussion_r1855134539


##########
mailbox/postgres/src/main/java/org/apache/james/mailbox/postgres/mail/dao/PostgresMailboxMessageDAO.java:
##########
@@ -117,78 +115,110 @@ public PostgresMailboxMessageDAO create(Optional<Domain> 
domain) {
 
     public static final SortField<Long> DEFAULT_SORT_ORDER_BY = 
MESSAGE_UID.asc();
 
-    private static SelectFinalStep<Record1<Long>> 
selectMessageUidByMailboxIdAndExtraConditionQuery(PostgresMailboxId mailboxId, 
Condition extraCondition, Limit limit, DSLContext dslContext) {
-        SelectSeekStep1<Record1<Long>, Long> queryWithoutLimit = 
dslContext.select(MESSAGE_UID)
-            .from(TABLE_NAME)
-            .where(MAILBOX_ID.eq((mailboxId.asUuid())))
-            .and(extraCondition)
-            .orderBy(MESSAGE_UID.asc());
-        return limit.getLimit().map(limitValue -> 
(SelectFinalStep<Record1<Long>>) queryWithoutLimit.limit(limitValue))
-            .orElse(queryWithoutLimit);
-    }
-
     private final PostgresExecutor postgresExecutor;
+    private final int queryBatchSize;
 
-    public PostgresMailboxMessageDAO(PostgresExecutor postgresExecutor) {
+    public PostgresMailboxMessageDAO(PostgresExecutor postgresExecutor,
+                                     int queryBatchSize) {
         this.postgresExecutor = postgresExecutor;
+        this.queryBatchSize = queryBatchSize;
     }
 
     public Mono<MessageUid> findFirstUnseenMessageUid(PostgresMailboxId 
mailboxId) {
-        return postgresExecutor.executeRow(dslContext -> 
Mono.from(selectMessageUidByMailboxIdAndExtraConditionQuery(mailboxId,
-                IS_SEEN.eq(false), Limit.limit(1), dslContext)))
+        return postgresExecutor.executeRow(dslContext -> 
Flux.from(dslContext.select(MESSAGE_UID)
+                .from(TABLE_NAME)
+                .where(MAILBOX_ID.eq((mailboxId.asUuid())))
+                .and(IS_SEEN.eq(false))
+                .orderBy(DEFAULT_SORT_ORDER_BY)
+                .limit(1)))
             .map(RECORD_TO_MESSAGE_UID_FUNCTION);
     }
 
     public Flux<MessageUid> listUnseen(PostgresMailboxId mailboxId) {
-        return postgresExecutor.executeRows(dslContext -> 
Flux.from(selectMessageUidByMailboxIdAndExtraConditionQuery(mailboxId,
-                IS_SEEN.eq(false), Limit.unlimited(), dslContext)))
-            .map(RECORD_TO_MESSAGE_UID_FUNCTION);
+        return Flux.range(0, OFFSET_UNLIMITED)
+            .concatMap(offsetIndex -> listUnseenBatch(mailboxId, offsetIndex * 
queryBatchSize, queryBatchSize))
+            .takeUntil(List::isEmpty)
+            .flatMapIterable(Function.identity());
+    }
+
+    private Mono<List<MessageUid>> listUnseenBatch(PostgresMailboxId 
mailboxId, int offset, int batchSize) {
+        return postgresExecutor.executeRows(dslContext -> 
Flux.from(dslContext.select(MESSAGE_UID)

Review Comment:
   No Really for UID I strongly believe .collectList would be better.
   
   > the batch help we early release connection to pool
   
   Wrong assumption: yes this wont lock one connection the entire time but we'd 
have many request: usage would be higher.



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