[fix](plan) only scan node with limit and no predicate can reduce to …

…1 instance (#31342)

This PR #25952 introduce a opt that if a scan node has limit and predicates, use only 1 instance to save cup and memory.
But this is wrong because we can not guarantee that the predicates can truly help to prune the data.
So I modify the logic to remove this opt.
Now, only scan node with limit and NO predicate can reduce to only 1 instance.

fe/fe-core/src/main/java/org/apache/doris/planner/OlapScanNode.java

@@ -1378,6 +1378,8 @@ public boolean getShouldColoScan() {
     // If scan is key search, should not enable the shared scan opt to prevent the performance problem
     // 1. where contain the eq or in expr of key column slot
     // 2. key column slot is distribution column and first column
+    // FIXME: this is not a good check, we can not guarantee that the predicate we check can truly
+    // help to prune the data, so we should check the predicate's effect on the data.
     protected boolean isKeySearch() {
         List<SlotRef> whereSlot = Lists.newArrayList();
         for (Expr conjunct : conjuncts) {

fe/fe-core/src/main/java/org/apache/doris/planner/ScanNode.java

@@ -735,7 +735,7 @@ public int getScanRangeNum() {
         return Integer.MAX_VALUE;
     }
 
-    public boolean haveLimitAndConjunts() {
-        return hasLimit() && !conjuncts.isEmpty();
+    public boolean shouldUseOneInstance() {
+        return hasLimit() && conjuncts.isEmpty();
     }
 }

fe/fe-core/src/main/java/org/apache/doris/qe/Coordinator.java

@@ -2067,9 +2067,9 @@ private void computeFragmentHosts() throws Exception {
                         if (node.isPresent() && (!node.get().shouldDisableSharedScan(context)
                                 || ignoreStorageDataDistribution)) {
                             expectedInstanceNum = Math.max(expectedInstanceNum, 1);
-                            // if have limit and conjunts, only need 1 instance to save cpu and
+                            // if have limit and no conjuncts, only need 1 instance to save cpu and
                             // mem resource
-                            if (node.get().haveLimitAndConjunts()) {
+                            if (node.get().shouldUseOneInstance()) {
                                 expectedInstanceNum = 1;
                             }
 
@@ -2080,9 +2080,9 @@ private void computeFragmentHosts() throws Exception {
                                 //the scan instance num should not larger than the tablets num
                                 expectedInstanceNum = Math.min(perNodeScanRanges.size(), parallelExecInstanceNum);
                             }
-                            // if have limit and conjunts, only need 1 instance to save cpu and
+                            // if have limit and no conjuncts, only need 1 instance to save cpu and
                             // mem resource
-                            if (node.get().haveLimitAndConjunts()) {
+                            if (node.get().shouldUseOneInstance()) {
                                 expectedInstanceNum = 1;
                             }