[improvement](statistics)Use count as ndv for unique/agg olap table single key column (#27186)

Single key column of unique/agg olap table has the same value of count and ndv, for this kind of column,
don't need to calculate ndv, simply use count as ndv.

fe/fe-core/src/main/java/org/apache/doris/statistics/BaseAnalysisTask.java

@@ -72,8 +72,8 @@ public abstract class BaseAnalysisTask {
             + "${idxId} AS `idx_id`, "
             + "'${colId}' AS `col_id`, "
             + "NULL AS `part_id`, "
-            + "ROUND(COUNT(1) * ${scaleFactor}) AS `row_count`, "
-            + "ROUND(NDV(`${colName}`) * ${scaleFactor})  as `ndv`, "
+            + "${rowCount} AS `row_count`, "
+            + "${ndvFunction} as `ndv`, "
             + "ROUND(SUM(CASE WHEN `${colName}` IS NULL THEN 1 ELSE 0 END) * ${scaleFactor}) AS `null_count`, "
             + "${min} AS `min`, "
             + "${max} AS `max`, "

fe/fe-core/src/main/java/org/apache/doris/statistics/HMSAnalysisTask.java

@@ -145,6 +145,7 @@ public class HMSAnalysisTask extends BaseAnalysisTask {
             if (distributionColumns.size() == 1 && distributionColumns.contains(col.getName().toLowerCase())) {
                 bucketFlag = true;
                 sb.append(LINEAR_ANALYZE_TEMPLATE);
+                params.put("ndvFunction", "ROUND(NDV(`${colName}`) * ${scaleFactor})");
                 params.put("rowCount", "ROUND(count(1) * ${scaleFactor})");
             } else {
                 sb.append(DUJ1_ANALYZE_TEMPLATE);

fe/fe-core/src/main/java/org/apache/doris/statistics/OlapAnalysisTask.java

@@ -17,6 +17,7 @@
 
 package org.apache.doris.statistics;
 
+import org.apache.doris.catalog.KeysType;
 import org.apache.doris.catalog.MaterializedIndex;
 import org.apache.doris.catalog.OlapTable;
 import org.apache.doris.catalog.Partition;
@@ -129,21 +130,26 @@ public class OlapAnalysisTask extends BaseAnalysisTask {
             }
             StringSubstitutor stringSubstitutor = new StringSubstitutor(params);
             String sql;
-            // Single distribution column is not fit for DUJ1 estimator, use linear estimator.
-            Set<String> distributionColumns = tbl.getDistributionColumnNames();
-            if (distributionColumns.size() == 1 && distributionColumns.contains(col.getName().toLowerCase())) {
+            if (useLinearAnalyzeTemplate()) {
                 params.put("min", StatisticsUtil.quote(min));
                 params.put("max", StatisticsUtil.quote(max));
+                // For single unique key, use count as ndv.
+                if (isSingleUniqueKey()) {
+                    params.put("ndvFunction", String.valueOf(rowCount));
+                } else {
+                    params.put("ndvFunction", "ROUND(NDV(`${colName}`) * ${scaleFactor})");
+                }
                 sql = stringSubstitutor.replace(LINEAR_ANALYZE_TEMPLATE);
             } else {
                 params.put("dataSizeFunction", getDataSizeFunction(col, true));
                 sql = stringSubstitutor.replace(DUJ1_ANALYZE_TEMPLATE);
             }
             LOG.info("Sample for column [{}]. Total rows [{}], rows to sample [{}], scale factor [{}], "
-                    + "limited [{}], distribute column [{}], partition column [{}], key column [{}]",
+                    + "limited [{}], distribute column [{}], partition column [{}], key column [{}], "
+                    + "is single unique key [{}]",
                     col.getName(), params.get("rowCount"), rowsToSample, params.get("scaleFactor"),
                     limitFlag, tbl.isDistributionColumn(col.getName()),
-                    tbl.isPartitionColumn(col.getName()), col.isKey());
+                    tbl.isPartitionColumn(col.getName()), col.isKey(), isSingleUniqueKey());
             runQuery(sql, false);
         }
     }
@@ -278,4 +284,28 @@ public class OlapAnalysisTask extends BaseAnalysisTask {
         }
         return sampleRows;
     }
+
+    /**
+     * Check if the task should use linear analyze template.
+     * @return True for single unique key column and single distribution column.
+     */
+    protected boolean useLinearAnalyzeTemplate() {
+        if (isSingleUniqueKey()) {
+            return true;
+        }
+        Set<String> distributionColumns = tbl.getDistributionColumnNames();
+        return distributionColumns.size() == 1 && distributionColumns.contains(col.getName().toLowerCase());
+    }
+
+    /**
+     * Check if the olap table has a single unique key.
+     * @return True if the table has a single unique/agg key. False otherwise.
+     */
+    protected boolean isSingleUniqueKey() {
+        int keysNum = ((OlapTable) tbl).getKeysNum();
+        KeysType keysType = ((OlapTable) tbl).getKeysType();
+        return col.isKey()
+            && keysNum == 1
+            && (keysType.equals(KeysType.UNIQUE_KEYS) || keysType.equals(KeysType.AGG_KEYS));
+    }
 }