Fixed DROP INDEX IF EXISTS and added simple test

dbms/src/Storages/AlterCommands.cpp

@@ -293,8 +293,12 @@ void AlterCommand::apply(ColumnsDescription & columns_description, IndicesDescri
                 });
 
         if (erase_it == indices_description.indices.end())
+        {
+            if (if_exists)
+                return;
             throw Exception("Wrong index name. Cannot find index `" + index_name + "` to drop.",
-                    ErrorCodes::LOGICAL_ERROR);
+                            ErrorCodes::LOGICAL_ERROR);
+        }
 
         indices_description.indices.erase(erase_it);
     }

dbms/tests/queries/0_stateless/00953_indices_alter_exceptions.reference

+ok

dbms/tests/queries/0_stateless/00953_indices_alter_exceptions.sh

+#!/usr/bin/env bash
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+. $CURDIR/../shell_config.sh
+
+EXCEPTION_SUCCESS_TEXT=ok
+
+$CLICKHOUSE_CLIENT --query="DROP TABLE IF EXISTS test_drop_indices;"
+
+$CLICKHOUSE_CLIENT --query="CREATE TABLE test_drop_indices
+(
+        a       UInt32,
+        b       UInt32
+)
+ENGINE = MergeTree ORDER BY (a);"
+
+# Must throw exception
+EXCEPTION_TEXT="Cannot find index \`test_index\` to drop.."
+$CLICKHOUSE_CLIENT --query="ALTER TABLE test_drop_indices DROP INDEX test_index;" 2>&1 \
+    | grep -q "$EXCEPTION_TEXT" && echo "$EXCEPTION_SUCCESS_TEXT" || echo "Did not thrown an exception"
+
+# Must not throw an exception
+$CLICKHOUSE_CLIENT --query="ALTER TABLE test_drop_indices DROP INDEX IF EXISTS test_index"
+
+$CLICKHOUSE_CLIENT --query="DROP TABLE test_drop_indices;"
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