SERVER-57452 Remove the sort stage in retryable write pre-fetch as oplog buffer now allows unordered insert.

2 files changed, 9 insertions, 49 deletions

diff --git a/jstests/replsets/tenant_migration_retryable_write_retry.js b/jstests/replsets/tenant_migration_retryable_write_retry.js
index 52a392dc7ae..95bdb4a7494 100644
--- a/jstests/replsets/tenant_migration_retryable_write_retry.js
+++ b/jstests/replsets/tenant_migration_retryable_write_retry.js
@@ -239,8 +239,7 @@ assert.commandWorked(donorPrimary.getDB(kDbName).runCommand({
 }));
 
 // The aggregation pipeline will return an array of retryable writes oplog entries (pre-image/
-// post-image oplog entries included) with "ts" < "startFetchingTimestamp" and sorted in ascending
-// order of "ts".
+// post-image oplog entries included) with "ts" < "startFetchingTimestamp".
 const aggRes = donorPrimary.getDB("config").runCommand({
     aggregate: "transactions",
     pipeline: [
@@ -322,25 +321,7 @@ const aggRes = donorPrimary.getDB("config").runCommand({
             as: "history",
             depthField: "depthForTenantMigration"
         }},
-        // Sort the oplog entries in each oplog chain.
-        {$set: {
-            history: {$reverseArray: {$reduce: {
-                input: "$history",
-                initialValue: {$range: [0, {$size: "$history"}]},
-                in: {$concatArrays: [
-                    {$slice: ["$$value", "$$this.depthForTenantMigration"]},
-                    ["$$this"],
-                    {$slice: [
-                        "$$value",
-                        {$subtract: [
-                            {$add: ["$$this.depthForTenantMigration", 1]},
-                            {$size: "$history"},
-                        ]},
-                    ]},
-                ]},
-            }}},
-        }},
-        // Now that we have the whole sorted chain, filter out entries that occurred after
+        // Now that we have the whole chain, filter out entries that occurred after
         // `startFetchingTimestamp`, since these entries will be fetched during the oplog fetching
         // phase.
         {$set: {
@@ -429,11 +410,6 @@ assert.eq(
     sessionsOnDonor.reduce(
         (numOplogEntries, sessionOnDonor) => sessionOnDonor.numOplogEntries + numOplogEntries, 0));
 
-// Verify that the oplog docs are sorted in ascending order of "ts".
-for (let i = 1; i < aggRes.cursor.firstBatch.length; i++) {
-    assert.lt(0, bsonWoCompare(aggRes.cursor.firstBatch[i].ts, aggRes.cursor.firstBatch[i - 1].ts));
-}
-
 for (let sessionOnDonor of sessionsOnDonor) {
     // Find the returned oplog docs for the session.
     const docs = aggRes.cursor.firstBatch.filter(
diff --git a/src/mongo/db/repl/tenant_migration_util.cpp b/src/mongo/db/repl/tenant_migration_util.cpp
index 3abf2fb95ac..dd9934c68d8 100644
--- a/src/mongo/db/repl/tenant_migration_util.cpp
+++ b/src/mongo/db/repl/tenant_migration_util.cpp
@@ -312,27 +312,11 @@ createRetryableWritesOplogFetchingPipelineForTenantMigrations(
             .firstElement(),
         expCtx));
 
-    // 9. Sort the oplog entries in each oplog chain. The $reduce expression sorts the `history`
-    //    array in ascending `depthForTenantMigration` order. The $reverseArray expression will
-    //    give an array in ascending timestamp order.
-    stages.emplace_back(DocumentSourceAddFields::create(fromjson("{\
-                    history: {$reverseArray: {$reduce: {\
-                        input: '$history',\
-                        initialValue: {$range: [0, {$size: '$history'}]},\
-                        in: {$concatArrays: [\
-                            {$slice: ['$$value', '$$this.depthForTenantMigration']},\
-                            ['$$this'],\
-                            {$slice: [\
-                                '$$value',\
-                                {$subtract: [\
-                                    {$add: ['$$this.depthForTenantMigration', 1]},\
-                                    {$size: '$history'}]}]}]}}}}}"),
-                                                        expCtx));
-
-    // 10. Filter out all oplog entries from the `history` array that occur after
+    // 9. Filter out all oplog entries from the `history` array that occur after
     //    `startFetchingTimestamp`. Since the oplog fetching and application stages will already
     //    capture entries after `startFetchingTimestamp`, we only need the earlier part of the oplog
-    //    chain.
+    //    chain. We do not need to sort the history after this since we will put the fetched entries
+    //    into the oplog buffer collection, where entries are read in timestamp order.
     stages.emplace_back(DocumentSourceAddFields::create(fromjson("{\
                     history: {$filter: {\
                         input: '$history',\
@@ -340,13 +324,13 @@ createRetryableWritesOplogFetchingPipelineForTenantMigrations(
                                                                  "]}}}}"),
                                                         expCtx));
 
-    // 11. Combine the oplog entries.
+    // 10. Combine the oplog entries.
     stages.emplace_back(DocumentSourceAddFields::create(fromjson("{\
                         'history': {$concatArrays: [\
                             '$preImageOps', '$postImageOps', '$history']}}"),
                                                         expCtx));
 
-    // 12. Fetch the complete oplog entries. `completeOplogEntry` is expected to contain exactly one
+    // 11. Fetch the complete oplog entries. `completeOplogEntry` is expected to contain exactly one
     //     element.
     stages.emplace_back(DocumentSourceLookUp::createFromBson(
         Doc{{"$lookup",
@@ -358,11 +342,11 @@ createRetryableWritesOplogFetchingPipelineForTenantMigrations(
             .firstElement(),
         expCtx));
 
-    // 13. Unwind oplog entries in each chain to the top-level array.
+    // 12. Unwind oplog entries in each chain to the top-level array.
     stages.emplace_back(
         DocumentSourceUnwind::create(expCtx, "completeOplogEntry", false, boost::none));
 
-    // 14. Replace root.
+    // 13. Replace root.
     stages.emplace_back(DocumentSourceReplaceRoot::createFromBson(
         fromjson("{$replaceRoot: {newRoot: '$completeOplogEntry'}}").firstElement(), expCtx));