presto-main/src/main/java/io/prestosql/operator/scalar/MapKeys.java/MapKeys/getKeys
class MapKeys:    @TypeParameter("K")
    @TypeParameter("V")
    @SqlType("array(K)")
    public static Block getKeys(
            @TypeParameter("K") Type keyType,
            @SqlType("map(K,V)") Block block)
    {
        BlockBuilder blockBuilder = keyType.createBlockBuilder(null, block.getPositionCount() / 2);
        for (int i = 0; i < block.getPositionCount(); i += 2) {
            keyType.appendTo(block, i, blockBuilder);
        }
        return blockBuilder.build();
    }