[Optimize] Add session variable max_fetch_remote_schema_tablet_count to limit tablets size for remote schema fetch

fe/fe-core/src/main/java/org/apache/doris/catalog/OlapTable.java

@@ -101,6 +101,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -2761,6 +2762,55 @@ public List<Tablet> getAllTablets() throws AnalysisException {
         return tablets;
     }
 
+    // Get sample tablets for remote desc schema
+    // 1. Estimate tablets for a partition, 1 at least
+    // 2. Pick the partition sorted with id in desc order, greater id with the newest partition
+    // 3. Truncate to sampleSize
+    public List<Tablet> getSampleTablets(int sampleSize) {
+        List<Tablet> sampleTablets = new ArrayList<>();
+        // Filter partition with empty data
+        Collection<Partition> partitions = getPartitions()
+                .stream()
+                .filter(partition -> partition.getVisibleVersion() > Partition.PARTITION_INIT_VERSION)
+                .collect(Collectors.toList());
+        if (partitions.isEmpty()) {
+            return sampleTablets;
+        }
+        // 1. Estimate tablets for a partition, 1 at least
+        int estimatePartitionTablets = Math.max(sampleSize / partitions.size(), 1);
+
+        // 2. Sort the partitions by id in descending order (greater id means the newest partition)
+        List<Partition> sortedPartitions = partitions.stream().sorted(new Comparator<Partition>() {
+            @Override
+            public int compare(Partition p1, Partition p2) {
+                // compare with desc order
+                return Long.compare(p2.getId(), p1.getId());
+            }
+        }).collect(Collectors.toList());
+
+        // 3. Collect tablets from partitions
+        for (Partition partition : sortedPartitions) {
+            List<Tablet> targetTablets = new ArrayList<>(partition.getBaseIndex().getTablets());
+            Collections.shuffle(targetTablets);
+            if (!targetTablets.isEmpty()) {
+                // Ensure we do not exceed the available number of tablets
+                int tabletsToFetch = Math.min(targetTablets.size(), estimatePartitionTablets);
+                sampleTablets.addAll(targetTablets.subList(0, tabletsToFetch));
+            }
+
+            if (sampleTablets.size() >= sampleSize) {
+                break;
+            }
+        }
+
+        // 4. Truncate to sample size if needed
+        if (sampleTablets.size() > sampleSize) {
+            sampleTablets = sampleTablets.subList(0, sampleSize);
+        }
+
+        return sampleTablets;
+    }
+
     // During `getNextVersion` and `updateVisibleVersionAndTime` period,
     // the write lock on the table should be held continuously
     public void updateVisibleVersionAndTime(long visibleVersion, long visibleVersionTime) {

fe/fe-core/src/main/java/org/apache/doris/common/proc/RemoteIndexSchemaProcDir.java

@@ -24,6 +24,7 @@
 import org.apache.doris.catalog.Tablet;
 import org.apache.doris.common.AnalysisException;
 import org.apache.doris.common.util.FetchRemoteTabletSchemaUtil;
+import org.apache.doris.qe.ConnectContext;
 
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
@@ -62,7 +63,8 @@ public ProcResult fetchResult() throws AnalysisException {
         table.readLock();
         try {
             OlapTable olapTable = (OlapTable) table;
-            tablets = olapTable.getAllTablets();
+            // Get sample tablets for remote desc schema
+            tablets = olapTable.getSampleTablets(ConnectContext.get().getSessionVariable().maxFetchRemoteTabletCount);
         } finally {
             table.readUnlock();
         }

fe/fe-core/src/main/java/org/apache/doris/common/proc/RemoteIndexSchemaProcNode.java

@@ -23,11 +23,13 @@
 import org.apache.doris.catalog.Tablet;
 import org.apache.doris.common.AnalysisException;
 import org.apache.doris.common.util.FetchRemoteTabletSchemaUtil;
+import org.apache.doris.qe.ConnectContext;
 
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Lists;
 
+import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 
@@ -62,6 +64,13 @@ public ProcResult fetchResult() throws AnalysisException {
                 tablets.add(tablet);
             }
         }
+        // Get the maximum number of Remote Tablets that can be fetched
+        int maxFetchCount = ConnectContext.get().getSessionVariable().maxFetchRemoteTabletCount;
+        // If the number of tablets is greater than the maximum fetch count, randomly select maxFetchCount tablets
+        if (tablets.size() > maxFetchCount) {
+            Collections.shuffle(tablets);
+            tablets = tablets.subList(0, maxFetchCount);
+        }
         List<Column> remoteSchema = new FetchRemoteTabletSchemaUtil(tablets).fetch();
         this.schema.addAll(remoteSchema);
         return IndexSchemaProcNode.createResult(this.schema, this.bfColumns);

fe/fe-core/src/main/java/org/apache/doris/common/util/FetchRemoteTabletSchemaUtil.java

@@ -92,6 +92,8 @@ public List<Column> fetch() {
             Long backendId = entry.getKey();
             Set<Long> tabletIds = entry.getValue();
             Backend backend = Env.getCurrentEnv().getCurrentSystemInfo().getBackend(backendId);
+            LOG.debug("fetch schema from coord backend {}, sample tablets count {}",
+                            backend.getId(), tabletIds.size());
             // only need alive be
             if (!backend.isAlive()) {
                 continue;

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

@@ -615,6 +615,8 @@ public class SessionVariable implements Serializable, Writable {
 
     public static final String FETCH_REMOTE_SCHEMA_TIMEOUT_SECONDS = "fetch_remote_schema_timeout_seconds";
 
+    public static final String MAX_FETCH_REMOTE_TABLET_COUNT = "max_fetch_remote_schema_tablet_count";
+
     // CLOUD_VARIABLES_BEGIN
     public static final String CLOUD_CLUSTER = "cloud_cluster";
     public static final String DISABLE_EMPTY_PARTITION_PRUNE = "disable_empty_partition_prune";
@@ -1911,6 +1913,9 @@ public void setIgnoreShapePlanNodes(String ignoreShapePlanNodes) {
     // fetch remote schema rpc timeout
     @VariableMgr.VarAttr(name = FETCH_REMOTE_SCHEMA_TIMEOUT_SECONDS, fuzzy = true)
     public long fetchRemoteSchemaTimeoutSeconds = 120;
+    // max tablet count for fetch remote schema
+    @VariableMgr.VarAttr(name = MAX_FETCH_REMOTE_TABLET_COUNT, fuzzy = true)
+    public int maxFetchRemoteTabletCount = 512;
 
     @VariableMgr.VarAttr(
             name = ENABLE_JOIN_SPILL,

regression-test/data/variant_p0/desc.out

@@ -198,3 +198,11 @@ v.金额	SMALLINT	Yes	false	\N	NONE
 k	BIGINT	Yes	true	\N	
 v	VARIANT	Yes	false	\N	NONE
 
+-- !sql15 --
+k	BIGINT	Yes	true	\N	
+v	VARIANT	Yes	false	\N	NONE
+v.a	TINYINT	Yes	false	\N	NONE
+v.b	TINYINT	Yes	false	\N	NONE
+v.c	TINYINT	Yes	false	\N	NONE
+v.d	TINYINT	Yes	false	\N	NONE
+

regression-test/suites/variant_p0/desc.groovy

@@ -238,6 +238,26 @@ suite("regression_test_variant_desc", "nonConcurrent"){
         sql """ insert into ${table_name} values (0, '100')"""
         sql """set describe_extend_variant_column = true"""
         qt_sql_12 """desc ${table_name}"""
+
+
+        // desc with large tablets
+        table_name = "large_tablets"
+        create_table_partition.call(table_name, "200") 
+        sql """insert into large_tablets values (1, '{"a" : 10}')"""
+        sql """insert into large_tablets values (3001, '{"b" : 10}')"""
+        sql """insert into large_tablets values (50001, '{"c" : 10}')"""
+        sql """insert into large_tablets values (99999, '{"d" : 10}')"""
+        sql """set max_fetch_remote_schema_tablet_count = 2"""
+        sql "desc large_tablets"
+        sql """set max_fetch_remote_schema_tablet_count = 128"""
+        sql "desc large_tablets"
+        sql """set max_fetch_remote_schema_tablet_count = 512"""
+        sql "desc large_tablets"
+        sql """set max_fetch_remote_schema_tablet_count = 2048"""
+        qt_sql15 "desc large_tablets"
+
+        sql "truncate table large_tablets"
+        sql "desc large_tablets"
     } finally {
         // reset flags
         set_be_config.call("variant_ratio_of_defaults_as_sparse_column", "0.95")