[pipeline](exec) disable shared scan in default and disable shared sc…

…an in limit with where scan (apache#25952)

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -1606,9 +1606,6 @@ public class Config extends ConfigBase {
     @ConfField(mutable = true, varType = VariableAnnotation.EXPERIMENTAL)
     public static boolean enable_cpu_hard_limit = false;
 
-    @ConfField(mutable = true)
-    public static boolean disable_shared_scan = false;
-
     @ConfField(mutable = false, masterOnly = true)
     public static int backend_rpc_timeout_ms = 60000; // 1 min
 

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

@@ -1267,11 +1267,9 @@ public String getNodeExplainString(String prefix, TExplainLevel detailLevel) {
     public int getNumInstances() {
         // In pipeline exec engine, the instance num equals be_num * parallel instance.
         // so here we need count distinct be_num to do the work. make sure get right instance
-        if (ConnectContext.get().getSessionVariable().getEnablePipelineEngine()) {
-            int parallelInstance = ConnectContext.get().getSessionVariable().getParallelExecInstanceNum();
-            long numBackend = scanRangeLocations.stream().flatMap(rangeLoc -> rangeLoc.getLocations().stream())
-                    .map(loc -> loc.backend_id).distinct().count();
-            return (int) (parallelInstance * numBackend);
+        if (ConnectContext.get().getSessionVariable().getEnablePipelineEngine()
+                && ConnectContext.get().getSessionVariable().getEnableSharedScan()) {
+            return ConnectContext.get().getSessionVariable().getParallelExecInstanceNum();
         }
         return scanRangeLocations.size();
     }
@@ -1328,7 +1326,7 @@ 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
-    public boolean isKeySearch() {
+    protected boolean isKeySearch() {
         List<SlotRef> whereSlot = Lists.newArrayList();
         for (Expr conjunct : conjuncts) {
             if (conjunct instanceof BinaryPredicate) {

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

@@ -43,6 +43,7 @@
 import org.apache.doris.common.UserException;
 import org.apache.doris.nereids.glue.translator.PlanTranslatorContext;
 import org.apache.doris.planner.external.FederationBackendPolicy;
+import org.apache.doris.qe.ConnectContext;
 import org.apache.doris.spi.Split;
 import org.apache.doris.statistics.StatisticalType;
 import org.apache.doris.statistics.query.StatsDelta;
@@ -148,6 +149,13 @@ protected Expr castToSlot(SlotDescriptor slotDesc, Expr expr) throws UserExcepti
      */
     public abstract List<TScanRangeLocations> getScanRangeLocations(long maxScanRangeLength);
 
+    // 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
+    protected boolean isKeySearch() {
+        return false;
+    }
+
     /**
      * Update required_slots in scan node contexts. This is called after Nereids planner do the projection.
      * In the projection process, some slots may be removed. So call this to update the slots info.
@@ -653,7 +661,14 @@ public static TScanRangeLocations createSingleScanRangeLocations(FederationBacke
         return scanRangeLocation;
     }
 
-    public boolean isKeySearch() {
-        return false;
+    // some scan should not enable the shared scan opt to prevent the performance problem
+    // 1. is key search
+    // 2. session variable not enable_shared_scan
+    public boolean shouldDisableSharedScan(ConnectContext context) {
+        return isKeySearch() || !context.getSessionVariable().getEnableSharedScan();
+    }
+
+    public boolean haveLimitAndConjunts() {
+        return hasLimit() && !conjuncts.isEmpty();
     }
 }

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

@@ -1955,27 +1955,37 @@ private void computeFragmentHosts() throws Exception {
 
                         // disable shared scan optimization if one of conditions below is met:
                         // 1. Use non-pipeline or pipelineX engine
-                        // 2. Number of scan ranges is larger than instances
-                        // 3. This fragment has a colocated scan node
-                        // 4. This fragment has a FileScanNode
-                        // 5. Disable shared scan optimization by session variable
-                        if (!enablePipelineEngine || perNodeScanRanges.size() > parallelExecInstanceNum
-                                || (node.isPresent() && node.get().getShouldColoScan())
+                        // 2. This fragment has a colocated scan node
+                        // 3. This fragment has a FileScanNode
+                        // 4. Disable shared scan optimization by session variable
+                        if (!enablePipelineEngine || (node.isPresent() && node.get().getShouldColoScan())
                                 || (node.isPresent() && node.get() instanceof FileScanNode)
-                                || (node.isPresent() && node.get().isKeySearch())
-                                || Config.disable_shared_scan || enablePipelineXEngine) {
+                                || (node.isPresent() && node.get().shouldDisableSharedScan(context))
+                                || enablePipelineXEngine) {
                             int expectedInstanceNum = 1;
                             if (parallelExecInstanceNum > 1) {
                                 //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
+                            // mem resource
+                            if (node.isPresent() && node.get().haveLimitAndConjunts()) {
+                                expectedInstanceNum = 1;
+                            }
+
                             perInstanceScanRanges = ListUtil.splitBySize(perNodeScanRanges,
                                     expectedInstanceNum);
                             sharedScanOpts = Collections.nCopies(perInstanceScanRanges.size(), false);
                         } else {
                             int expectedInstanceNum = Math.min(parallelExecInstanceNum,
                                     leftMostNode.getNumInstances());
                             expectedInstanceNum = Math.max(expectedInstanceNum, 1);
+                            // if have limit and conjunts, only need 1 instance to save cpu and
+                            // mem resource
+                            if (node.isPresent() && node.get().haveLimitAndConjunts()) {
+                                expectedInstanceNum = 1;
+                            }
+
                             perInstanceScanRanges = Collections.nCopies(expectedInstanceNum, perNodeScanRanges);
                             sharedScanOpts = Collections.nCopies(perInstanceScanRanges.size(), true);
                         }

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

@@ -214,6 +214,8 @@ public class SessionVariable implements Serializable, Writable {
 
     public static final String ENABLE_PIPELINE_X_ENGINE = "enable_pipeline_x_engine";
 
+    public static final String ENABLE_SHARED_SCAN = "enable_shared_scan";
+
     public static final String ENABLE_LOCAL_SHUFFLE = "enable_local_shuffle";
 
     public static final String ENABLE_AGG_STATE = "enable_agg_state";
@@ -738,6 +740,11 @@ public class SessionVariable implements Serializable, Writable {
 
     @VariableMgr.VarAttr(name = ENABLE_PIPELINE_X_ENGINE, fuzzy = false, varType = VariableAnnotation.EXPERIMENTAL)
     private boolean enablePipelineXEngine = false;
+
+    @VariableMgr.VarAttr(name = ENABLE_SHARED_SCAN, fuzzy = false, varType = VariableAnnotation.EXPERIMENTAL,
+            needForward = true)
+    private boolean enableSharedScan = false;
+
     @VariableMgr.VarAttr(name = ENABLE_LOCAL_SHUFFLE, fuzzy = false, varType = VariableAnnotation.EXPERIMENTAL)
     private boolean enableLocalShuffle = false;
 
@@ -2832,6 +2839,10 @@ public boolean getEnablePipelineEngine() {
         return enablePipelineEngine || enablePipelineXEngine;
     }
 
+    public boolean getEnableSharedScan() {
+        return enableSharedScan;
+    }
+
     public boolean getEnablePipelineXEngine() {
         return enablePipelineXEngine;
     }