enh: optimize last pruning logic

source/dnode/vnode/src/tsdb/tsdbCacheRead.c

@@ -332,7 +332,7 @@ int32_t tsdbRetrieveCacheRows(void* pReader, SSDataBlock* pResBlock, const int32
   // retrieve the only one last row of all tables in the uid list.
   if (HASTYPE(pr->type, CACHESCAN_RETRIEVE_TYPE_SINGLE)) {
     int64_t st = taosGetTimestampUs();
-    int64_t totalMinTs = INT64_MAX;
+    int64_t totalLastTs = INT64_MAX;
     for (int32_t i = 0; i < pr->numOfTables; ++i) {
       STableKeyInfo* pKeyInfo = &pr->pTableList[i];
 
@@ -351,7 +351,7 @@ int32_t tsdbRetrieveCacheRows(void* pReader, SSDataBlock* pResBlock, const int32
 
       {
         bool    hasNotNullRow = true;
-        int64_t minTs = INT64_MAX;
+        int64_t singleTableLastTs = INT64_MAX;
         for (int32_t k = 0; k < pr->numOfCols; ++k) {
           int32_t slotId = slotIds[k];
 
@@ -362,7 +362,7 @@ int32_t tsdbRetrieveCacheRows(void* pReader, SSDataBlock* pResBlock, const int32
               hasRes = true;
               p->ts = pCol->ts;
               p->colVal = pCol->colVal;
-              minTs = pCol->ts;
+              singleTableLastTs = pCol->ts;
 
               // only set value for last row query
               if (HASTYPE(pr->type, CACHESCAN_RETRIEVE_LAST_ROW)) {
@@ -387,8 +387,8 @@ int32_t tsdbRetrieveCacheRows(void* pReader, SSDataBlock* pResBlock, const int32
 
               hasRes = true;
               p->ts = pColVal->ts;
-              if (pColVal->ts < minTs && HASTYPE(pr->type, CACHESCAN_RETRIEVE_LAST)) {
-                minTs = pColVal->ts;
+              if (pColVal->ts < singleTableLastTs && HASTYPE(pr->type, CACHESCAN_RETRIEVE_LAST)) {
+                singleTableLastTs = pColVal->ts;
               }
 
               if (!IS_VAR_DATA_TYPE(pColVal->colVal.type)) {
@@ -408,12 +408,12 @@ int32_t tsdbRetrieveCacheRows(void* pReader, SSDataBlock* pResBlock, const int32
         }
 
         if (hasNotNullRow) {
-          if (totalMinTs > minTs) {
-            totalMinTs = minTs;
+          if (INT64_MAX == totalLastTs || totalLastTs < singleTableLastTs) {
+            totalLastTs = singleTableLastTs;
           }
           double cost = (taosGetTimestampUs() - st) / 1000.0;
           if (cost > tsCacheLazyLoadThreshold) {
-            pr->lastTs = totalMinTs;
+            pr->lastTs = totalLastTs;
           }
         }
       }