Add test

tests/queries/0_stateless/01576_alter_low_cardinality_and_select.reference

+733	733
+CREATE TABLE default.alter_table\n(\n    `key` UInt64,\n    `value` LowCardinality(String)\n)\nENGINE = MergeTree\nORDER BY key\nSETTINGS index_granularity = 8192
+all_1_1_0
+all_2_2_0
+all_3_3_0
+701	701

tests/queries/0_stateless/01576_alter_low_cardinality_and_select.sh

+#!/usr/bin/env bash
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+. "$CURDIR"/../shell_config.sh
+
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS alter_table"
+
+${CLICKHOUSE_CLIENT} --query "CREATE TABLE alter_table (key UInt64, value String) ENGINE MergeTree ORDER BY key"
+
+# we don't need mutations and merges
+${CLICKHOUSE_CLIENT} --query "SYSTEM STOP MERGES alter_table"
+
+${CLICKHOUSE_CLIENT} --query "INSERT INTO alter_table SELECT number, toString(number) FROM numbers(10000)"
+${CLICKHOUSE_CLIENT} --query "INSERT INTO alter_table SELECT number, toString(number) FROM numbers(10000, 10000)"
+${CLICKHOUSE_CLIENT} --query "INSERT INTO alter_table SELECT number, toString(number) FROM numbers(20000, 10000)"
+
+${CLICKHOUSE_CLIENT} --query "SELECT * FROM alter_table WHERE value == '733'"
+
+${CLICKHOUSE_CLIENT} --query "ALTER TABLE alter_table MODIFY COLUMN value LowCardinality(String)" &
+
+type_query="SELECT type FROM system.columns WHERE name = 'value' and table='alter_table' and database='${CLICKHOUSE_DATABASE}'"
+value_type=""
+
+# waiting until schema will change (but not data)
+while [[ "$value_type" != "LowCardinality(String)" ]]
+do
+    sleep 0.1
+    value_type=$($CLICKHOUSE_CLIENT --query "$type_query")
+done
+
+# checking type is LowCardinalty
+${CLICKHOUSE_CLIENT} --query "SHOW CREATE TABLE alter_table"
+
+# checking no mutations happened
+${CLICKHOUSE_CLIENT} --query "SELECT name FROM system.parts where table='alter_table' and active and database='${CLICKHOUSE_DATABASE}' ORDER BY name"
+
+# checking that conversions applied "on fly" works
+${CLICKHOUSE_CLIENT} --query "SELECT * FROM alter_table PREWHERE key > 700 WHERE value = '701'"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS alter_table"