SERVER-56509 Wrap unique index insertion _keyExists call in a WT cursor reconfigure

This improves, but does not entirely fix a performance regression with
inserts and updates on unique indexes

(cherry picked from commit c5ac2eb1ea145693e1c6b974e88a2cfc18780134)

2 files changed, 17 insertions, 13 deletions

diff --git a/jstests/noPassthrough/profile_operation_metrics.js b/jstests/noPassthrough/profile_operation_metrics.js
index 83a030af2f3..39427500a6d 100644
--- a/jstests/noPassthrough/profile_operation_metrics.js
+++ b/jstests/noPassthrough/profile_operation_metrics.js
@@ -694,9 +694,8 @@ const operations = [
             // Insert should not perform any reads.
             assert.eq(profileDoc.docBytesRead, 0);
             assert.eq(profileDoc.docUnitsRead, 0);
-            // Reads the index entry for 'a' to determine uniqueness.
-            assert.eq(profileDoc.idxEntryBytesRead, 6);
-            assert.eq(profileDoc.idxEntryUnitsRead, 1);
+            assert.eq(profileDoc.idxEntryBytesRead, 0);
+            assert.eq(profileDoc.idxEntryUnitsRead, 0);
             assert.eq(profileDoc.cursorSeeks, 1);
             assert.eq(profileDoc.docBytesWritten, 29);
             assert.eq(profileDoc.docUnitsWritten, 1);
@@ -762,9 +761,9 @@ const operations = [
                 assert.gte(profileDoc.docUnitsRead, 1);
                 assert.gte(profileDoc.cursorSeeks, 4);
             }
-            // Reads index entries on '_id' for the lookup and 'a' to ensure uniqueness.
-            assert.eq(profileDoc.idxEntryBytesRead, 9);
-            assert.eq(profileDoc.idxEntryUnitsRead, 2);
+            // Reads index entries on '_id' for the lookup.
+            assert.eq(profileDoc.idxEntryBytesRead, 3);
+            assert.eq(profileDoc.idxEntryUnitsRead, 1);
             // This out-of-place update should perform a direct insert because it is not large
             // enough to qualify for the in-place update path.
             assert.eq(profileDoc.docBytesWritten, 29);
@@ -802,9 +801,9 @@ const operations = [
                 assert.gte(profileDoc.docUnitsRead, 9);
                 assert.gte(profileDoc.cursorSeeks, 4);
             }
-            // Reads index entries on '_id' for the lookup and 'a' to ensure uniqueness.
-            assert.eq(profileDoc.idxEntryBytesRead, 10);
-            assert.eq(profileDoc.idxEntryUnitsRead, 2);
+            // Reads index entries on '_id'.
+            assert.eq(profileDoc.idxEntryBytesRead, 4);
+            assert.eq(profileDoc.idxEntryUnitsRead, 1);
             if (FixtureHelpers.isReplSet(db)) {
                 // When WT_MODIFY is used on a replicated collection fewer bytes are written per the
                 // comment about WT_MODIFY above.
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
index 690fbc0d0d8..01a7d64f72f 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
@@ -1408,13 +1408,13 @@ bool WiredTigerIndexUnique::_keyExists(OperationContext* opCtx,
     int cmp;
     int ret = wiredTigerPrepareConflictRetry(opCtx, [&] { return c->search_near(c, &cmp); });
 
+    auto& metricsCollector = ResourceConsumption::MetricsCollector::get(opCtx);
+    metricsCollector.incrementOneCursorSeek();
+
     if (ret == WT_NOTFOUND)
         return false;
     invariantWTOK(ret);
 
-    auto& metricsCollector = ResourceConsumption::MetricsCollector::get(opCtx);
-    metricsCollector.incrementOneCursorSeek();
-
     if (cmp == 0)
         return true;
 
@@ -1565,7 +1565,12 @@ Status WiredTigerIndexUnique::_insert(OperationContext* opCtx,
         invariantWTOK(ret);
 
         // Second phase looks up for existence of key to avoid insertion of duplicate key
-        if (_keyExists(opCtx, c, keyString.getBuffer(), sizeWithoutRecordId)) {
+        // The usage of 'prefix_key=true' enables an optimization that allows this search to return
+        // more quickly. See SERVER-56509.
+        c->reconfigure(c, "prefix_key=true");
+        ON_BLOCK_EXIT([c] { c->reconfigure(c, "prefix_key=false"); });
+        auto keyExists = _keyExists(opCtx, c, keyString.getBuffer(), sizeWithoutRecordId);
+        if (keyExists) {
             auto key = KeyString::toBson(
                 keyString.getBuffer(), sizeWithoutRecordId, _ordering, keyString.getTypeBits());
             auto entry = _desc->getEntry();