Optimize ZUNION[STORE] command by removing unnecessary accumulator di…

…ct (valkey-io#829)

In the past implementation of `ZUNION` and `ZUNIONSTORE` commands, we
first create a temporary dict called `accumulator`. After adding all
member-score mappings to `accumulator`, we still need to convert
`accumulator` back to the final dict `dstzset->dict`. However, we can
directly use `dstzset->dict` to avoid the additional copy operation.

This PR removes the `accumulator` dict and directly uses` dstzset->dict
`to store the member-score mappings.

- **Test**
First, I added 300 unique elements to two sorted sets called
'zunion_test1' and 'zunion_test2'. Then, I tested `zunion` and
`zunionstore` on these two sorted sets. The test results shown below
indicate that the performance of both zunion and zunionstore improved
about 31%.

### ZUNION
#### unstable
```
./valkey-benchmark -P 10 -n 100000 zunion 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 2713.41 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      146.252     3.464   153.343   182.015   184.959   192.895
```
#### This PR
```
./valkey-benchmark -P 10 -n 100000 zunion 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 3543.84 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      108.259     2.984   114.239   141.695   145.151   160.255
```
### ZUNIONSTORE
#### unstable
```
./valkey-benchmark -P 10 -n 100000 zunionstore out 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 3168.07 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      157.511     3.368   183.167   189.311   193.535   231.679
```
#### This PR
```
./valkey-benchmark -P 10 -n 100000 zunionstore out 2 zunion_test1 zunion_test2

Summary:
  throughput summary: 4144.73 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
      120.374     2.648   141.823   149.119   153.855   183.167
```

---------

Signed-off-by: RayCao <[email protected]>
Signed-off-by: zisong.cw <[email protected]>
Signed-off-by: mwish <[email protected]>

src/t_zset.c

@@ -2529,15 +2529,6 @@ static void zdiff(zsetopsrc *src, long setnum, zset *dstzset, size_t *maxelelen,
     }
 }
 
-dictType setAccumulatorDictType = {
-    dictSdsHash,       /* hash function */
-    NULL,              /* key dup */
-    dictSdsKeyCompare, /* key compare */
-    NULL,              /* key destructor */
-    NULL,              /* val destructor */
-    NULL               /* allow to expand */
-};
-
 /* The zunionInterDiffGenericCommand() function is called in order to implement the
  * following commands: ZUNION, ZINTER, ZDIFF, ZUNIONSTORE, ZINTERSTORE, ZDIFFSTORE,
  * ZINTERCARD.
@@ -2724,15 +2715,14 @@ void zunionInterDiffGenericCommand(client *c, robj *dstkey, int numkeysIndex, in
             zuiClearIterator(&src[0]);
         }
     } else if (op == SET_OP_UNION) {
-        dict *accumulator = dictCreate(&setAccumulatorDictType);
         dictIterator *di;
         dictEntry *de, *existing;
         double score;
 
         if (setnum) {
             /* Our union is at least as large as the largest set.
              * Resize the dictionary ASAP to avoid useless rehashing. */
-            dictExpand(accumulator, zuiLength(&src[setnum - 1]));
+            dictExpand(dstzset->dict, zuiLength(&src[setnum - 1]));
         }
 
         /* Step 1: Create a dictionary of elements -> aggregated-scores
@@ -2747,7 +2737,7 @@ void zunionInterDiffGenericCommand(client *c, robj *dstkey, int numkeysIndex, in
                 if (isnan(score)) score = 0;
 
                 /* Search for this element in the accumulating dictionary. */
-                de = dictAddRaw(accumulator, zuiSdsFromValue(&zval), &existing);
+                de = dictAddRaw(dstzset->dict, zuiSdsFromValue(&zval), &existing);
                 /* If we don't have it, we need to create a new entry. */
                 if (!existing) {
                     tmp = zuiNewSdsFromValue(&zval);
@@ -2757,7 +2747,7 @@ void zunionInterDiffGenericCommand(client *c, robj *dstkey, int numkeysIndex, in
                     totelelen += sdslen(tmp);
                     if (sdslen(tmp) > maxelelen) maxelelen = sdslen(tmp);
                     /* Update the element with its initial score. */
-                    dictSetKey(accumulator, de, tmp);
+                    dictSetKey(dstzset->dict, de, tmp);
                     dictSetDoubleVal(de, score);
                 } else {
                     /* Update the score with the score of the new instance
@@ -2774,21 +2764,15 @@ void zunionInterDiffGenericCommand(client *c, robj *dstkey, int numkeysIndex, in
         }
 
         /* Step 2: convert the dictionary into the final sorted set. */
-        di = dictGetIterator(accumulator);
-
-        /* We now are aware of the final size of the resulting sorted set,
-         * let's resize the dictionary embedded inside the sorted set to the
-         * right size, in order to save rehashing time. */
-        dictExpand(dstzset->dict, dictSize(accumulator));
+        di = dictGetIterator(dstzset->dict);
 
         while ((de = dictNext(di)) != NULL) {
             sds ele = dictGetKey(de);
             score = dictGetDoubleVal(de);
             znode = zslInsert(dstzset->zsl, score, ele);
-            dictAdd(dstzset->dict, ele, &znode->score);
+            dictSetVal(dstzset->dict, de, &znode->score);
         }
         dictReleaseIterator(di);
-        dictRelease(accumulator);
     } else if (op == SET_OP_DIFF) {
         zdiff(src, setnum, dstzset, &maxelelen, &totelelen);
     } else {