Data Model

Data Model

This topic introduces the data models in Doris from a logical perspective so you can make better use of Doris in different business scenarios.

Basic Concepts

In Doris, data is logically described in the form of tables. A table consists of rows and columns. Row is a row of user data. Column is used to describe different fields in a row of data.

Columns can be divided into two categories: Key and Value. From a business perspective, Key and Value correspond to dimension columns and indicator columns, respectively. The key column of Doris is the column specified in the table creation statement. The column after the keyword 'unique key' or 'aggregate key' or 'duplicate key' in the table creation statement is the key column, and the rest except the key column is the value column .

Data models in Doris fall into three types:

• Aggregate
• Unique
• Duplicate

The following is the detailed introduction to each of them.

Aggregate Model

We illustrate what aggregation model is and how to use it correctly with practical examples.

Example 1: Importing Data Aggregation

Assume that the business has the following data table schema:

ColumnName	Type	AggregationType	Comment
userid	LARGEINT		user id
date	DATE		date of data filling
City	VARCHAR (20)		User City
age	SMALLINT		User age
sex	TINYINT		User gender
Last_visit_date	DATETIME	REPLACE	Last user access time
Cost	BIGINT	SUM	Total User Consumption
max dwell time	INT	MAX	Maximum user residence time
min dwell time	INT	MIN	User minimum residence time

The corresponding to CREATE TABLE statement would be as follows (omitting the Partition and Distribution information):

CREATE DATABASE IF NOT EXISTS example_db;

CREATE TABLE IF NOT EXISTS example_db.example_tbl_agg1
(
    `user_id` LARGEINT NOT NULL COMMENT "user id",
    `date` DATE NOT NULL COMMENT "data import time",
    `city` VARCHAR(20) COMMENT "city",
    `age` SMALLINT COMMENT "age",
    `sex` TINYINT COMMENT "gender",
    `last_visit_date` DATETIME REPLACE DEFAULT "1970-01-01 00:00:00" COMMENT "last visit date time",
    `cost` BIGINT SUM DEFAULT "0" COMMENT "user total cost",
    `max_dwell_time` INT MAX DEFAULT "0" COMMENT "user max dwell time",
    `min_dwell_time` INT MIN DEFAULT "99999" COMMENT "user min dwell time"
)
AGGREGATE KEY(`user_id`, `date`, `city`, `age`, `sex`)
DISTRIBUTED BY HASH(`user_id`) BUCKETS 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1"
);

As you can see, this is a typical fact table of user information and visit behaviors. In star models, user information and visit behaviors are usually stored in dimension tables and fact tables, respectively. Here, for the convenience of explanation, we store the two types of information in one single table.

The columns in the table are divided into Key (dimension) columns and Value (indicator columns) based on whether they are set with an AggregationType. Key columns are not set with an AggregationType, such as user_id, date, and age, while Value columns are.

When data are imported, rows with the same contents in the Key columns will be aggregated into one row, and their values in the Value columns will be aggregated as their AggregationType specify. Currently, there are several aggregation methods and "agg_state" options available:

1. SUM: Accumulate the values in multiple rows.
2. REPLACE: The newly imported value will replace the previous value.
3. MAX: Keep the maximum value.
4. MIN: Keep the minimum value.
5. REPLACE_IF_NOT_NULL: Non-null value replacement. Unlike REPLACE, it does not replace null values.
6. HLL_UNION: Aggregation method for columns of HLL type, using the HyperLogLog algorithm for aggregation.
7. BITMAP_UNION: Aggregation method for columns of BITMAP type, performing a union aggregation of bitmaps.

If these aggregation methods cannot meet the requirements, you can choose to use the "agg_state" type.

Suppose that you have the following import data (raw data):

user_id	date	city	age	sex	last_visit_date	cost	max_dwell_time	min_dwell_time
10000	2017-10-01	Beijing	20	0	2017-10-01 06:00	20	10	10
10000	2017-10-01	Beijing	20	0	2017-10-01 07:00	15	2	2
10001	2017-10-01	Beijing	30	1	2017-10-01 17:05:45	2	22	22
10002	2017-10-02	Shanghai	20	1	2017-10-02 12:59:12	200	5	5
10003	2017-10-02	Guangzhou	32	0	2017-10-02 11:20:00	30	11	11
10004	2017-10-01	Shenzhen	35	0	2017-10-01 10:00:15	100	3	3
10004	2017-10-03	Shenzhen	35	0	2017-10-03 10:20:22	11	6	6

And you can import data with the following sql:

insert into example_db.example_tbl_agg1 values
(10000,"2017-10-01","Beijing",20,0,"2017-10-01 06:00:00",20,10,10),
(10000,"2017-10-01","Beijing",20,0,"2017-10-01 07:00:00",15,2,2),
(10001,"2017-10-01","Beijing",30,1,"2017-10-01 17:05:45",2,22,22),
(10002,"2017-10-02","Shanghai",20,1,"2017-10-02 12:59:12",200,5,5),
(10003,"2017-10-02","Guangzhou",32,0,"2017-10-02 11:20:00",30,11,11),
(10004,"2017-10-01","Shenzhen",35,0,"2017-10-01 10:00:15",100,3,3),
(10004,"2017-10-03","Shenzhen",35,0,"2017-10-03 10:20:22",11,6,6);

Assume that this is a table recording the user behaviors when visiting a certain commodity page. The first row of data, for example, is explained as follows:

Data	Description
10000	User id, each user uniquely identifies id
2017-10-01	Data storage time, accurate to date
Beijing	User City
20	User Age
0	Gender male (1 for female)
2017-10-01 06:00	User's time to visit this page, accurate to seconds
20	Consumption generated by the user's current visit
10	User's visit, time to stay on the page
10	User's current visit, time spent on the page (redundancy)

After this batch of data is imported into Doris correctly, it will be stored in Doris as follows:

user_id	date	city	age	sex	last_visit_date	cost	max_dwell_time	min_dwell_time
10000	2017-10-01	Beijing	20	0	2017-10-01 07:00	35	10	2
10001	2017-10-01	Beijing	30	1	2017-10-01 17:05:45	2	22	22
10002	2017-10-02	Shanghai	20	1	2017-10-02 12:59:12	200	5	5
10003	2017-10-02	Guangzhou	32	0	2017-10-02 11:20:00	30	11	11
10004	2017-10-01	Shenzhen	35	0	2017-10-01 10:00:15	100	3	3
10004	2017-10-03	Shenzhen	35	0	2017-10-03 10:20:22	11	6	6

As you can see, the data of User 10000 have been aggregated to one row, while those of other users remain the same. The explanation for the aggregated data of User 10000 is as follows (the first 5 columns remain unchanged, so it starts with Column 6 last_visit_date):

*2017-10-01 07:00: The last_visit_date column is aggregated by REPLACE, so 2017-10-01 07:00 has replaced 2017-10-01 06:00.

Note: When using REPLACE to aggregate data from the same import batch, the order of replacement is uncertain. That means, in this case, the data eventually saved in Doris could be 2017-10-01 06:00. However, for different import batches, it is certain that data from the new batch will replace those from the old batch.

*35: The costcolumn is aggregated by SUM, so the update value 35 is the result of 20 + 15.

*10: The max_dwell_time column is aggregated by MAX, so 10 is saved as it is the maximum between 10 and 2.

*2: The min_dwell_time column is aggregated by MIN, so 2 is saved as it is the minimum between 10 and 2.

After aggregation, Doris only stores the aggregated data. In other words, the detailed raw data will no longer be available.

Example 2: Keep Detailed Data

Here is a modified version of the table schema in Example 1:

ColumnName	Type	AggregationType	Comment
user_id	LARGEINT		User ID
date	DATE		Date when the data are imported
timestamp	DATETIME		Date and time when the data are imported (with second-level accuracy)
city	VARCHAR (20)		User location city
age	SMALLINT		User age
sex	TINYINT		User gender
last visit date	DATETIME	REPLACE	Last visit time of the user
cost	BIGINT	SUM	Total consumption of the user
max_dwell_time	INT	MAX	Maximum user dwell time
min_dwell_time	INT	MIN	Minimum user dwell time

A new column timestamp has been added to record the date and time when the data are imported (with second-level accuracy).

CREATE TABLE IF NOT EXISTS example_db.example_tbl_agg2
(
    `user_id` LARGEINT NOT NULL COMMENT "用户id",
    `date` DATE NOT NULL COMMENT "数据灌入日期时间",
    `timestamp` DATETIME NOT NULL COMMENT "数据灌入日期时间戳",
    `city` VARCHAR(20) COMMENT "用户所在城市",
    `age` SMALLINT COMMENT "用户年龄",
    `sex` TINYINT COMMENT "用户性别",
    `last_visit_date` DATETIME REPLACE DEFAULT "1970-01-01 00:00:00" COMMENT "用户最后一次访问时间",
    `cost` BIGINT SUM DEFAULT "0" COMMENT "用户总消费",
    `max_dwell_time` INT MAX DEFAULT "0" COMMENT "用户最大停留时间",
    `min_dwell_time` INT MIN DEFAULT "99999" COMMENT "用户最小停留时间"
)
AGGREGATE KEY(`user_id`, `date`, `timestamp` ,`city`, `age`, `sex`)
DISTRIBUTED BY HASH(`user_id`) BUCKETS 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1"
);

Suppose that the import data are as follows:

user_id	date	timestamp	city	age	sex	last_visit_date	cost	max_dwell_time	min_dwell_time
10000	2017-10-01	2017-10-01 08:00:05	Beijing	20	0	2017-10-01 06:00	20	10	10
10000	2017-10-01	2017-10-01 09:00:05	Beijing	20	0	2017-10-01 07:00	15	2	2
10001	2017-10-01	2017-10-01 18:12:10	Beijing	30	1	2017-10-01 17:05:45	2	22	22
10002	2017-10-02	2017-10-02 13:10:00	Shanghai	20	1	2017-10-02 12:59:12	200	5	5
10003	2017-10-02	2017-10-02 13:15:00	Guangzhou	32	0	2017-10-02 11:20:00	30	11	11
10004	2017-10-01	2017-10-01 12:12:48	Shenzhen	35	0	2017-10-01 10:00:15	100	3	3
10004	2017-10-03	2017-10-03 12:38:20	Shenzhen	35	0	2017-10-03 10:20:22	11	6	6

And you can import data with the following sql:

insert into example_db.example_tbl_agg2 values
(10000,"2017-10-01","2017-10-01 08:00:05","Beijing",20,0,"2017-10-01 06:00:00",20,10,10),
(10000,"2017-10-01","2017-10-01 09:00:05","Beijing",20,0,"2017-10-01 07:00:00",15,2,2),
(10001,"2017-10-01","2017-10-01 18:12:10","Beijing",30,1,"2017-10-01 17:05:45",2,22,22),
(10002,"2017-10-02","2017-10-02 13:10:00","Shanghai",20,1,"2017-10-02 12:59:12",200,5,5),
(10003,"2017-10-02","2017-10-02 13:15:00","Guangzhou",32,0,"2017-10-02 11:20:00",30,11,11),
(10004,"2017-10-01","2017-10-01 12:12:48","Shenzhen",35,0,"2017-10-01 10:00:15",100,3,3),
(10004,"2017-10-03","2017-10-03 12:38:20","Shenzhen",35,0,"2017-10-03 10:20:22",11,6,6);

After importing, this batch of data will be stored in Doris as follows:

user_id	date	timestamp	city	age	sex	last_visit_date	cost	max_dwell_time	min_dwell_time
10000	2017-10-01	2017-10-01 08:00:05	Beijing	20	0	2017-10-01 06:00	20	10	10
10000	2017-10-01	2017-10-01 09:00:05	Beijing	20	0	2017-10-01 07:00	15	2	2
10001	2017-10-01	2017-10-01 18:12:10	Beijing	30	1	2017-10-01 17:05:45	2	22	22
10002	2017-10-02	2017-10-02 13:10:00	Shanghai	20	1	2017-10-02 12:59:12	200	5	5
10003	2017-10-02	2017-10-02 13:15:00	Guangzhou	32	0	2017-10-02 11:20:00	30	11	11
10004	2017-10-01	2017-10-01 12:12:48	Shenzhen	35	0	2017-10-01 10:00:15	100	3	3
10004	2017-10-03	2017-10-03 12:38:20	Shenzhen	35	0	2017-10-03 10:20:22	11	6	6

As you can see, the stored data are exactly the same as the import data. No aggregation has ever happened. This is because, the newly added timestamp column results in difference of Keys among the rows. That is to say, as long as the Keys of the rows are not identical in the import data, Doris can save the complete detailed data even in the Aggregate Model.

Example 3: Aggregate Import Data and Existing Data

Based on Example 1, suppose that you have the following data stored in Doris:

user_id	date	city	age	sex	last_visit_date	cost	max_dwell_time	min_dwell_time
10000	2017-10-01	Beijing	20	0	2017-10-01 07:00	35	10	2
10001	2017-10-01	Beijing	30	1	2017-10-01 17:05:45	2	22	22
10002	2017-10-02	Shanghai	20	1	2017-10-02 12:59:12	200	5	5
10003	2017-10-02	Guangzhou	32	0	2017-10-02 11:20:00	30	11	11
10004	2017-10-01	Shenzhen	35	0	2017-10-01 10:00:15	100	3	3
10004	2017-10-03	Shenzhen	35	0	2017-10-03 10:20:22	11	6	6

Now you need to import a new batch of data:

user_id	date	city	age	sex	last_visit_date	cost	max_dwell_time	min_dwell_time
10004	2017-10-03	Shenzhen	35	0	2017-10-03 11:22:00	44	19	19
10005	2017-10-03	Changsha	29	1	2017-10-03 18:11:02	3	1	1

And you can import data with the following sql:

insert into example_db.example_tbl_agg1 values
(10004,"2017-10-03","Shenzhen",35,0,"2017-10-03 11:22:00",44,19,19),
(10005,"2017-10-03","Changsha",29,1,"2017-10-03 18:11:02",3,1,1);

After importing, the data stored in Doris will be updated as follows:

user_id	date	city	age	sex	last_visit_date	cost	max_dwell_time	min_dwell_time
10000	2017-10-01	Beijing	20	0	2017-10-01 07:00	35	10	2
10001	2017-10-01	Beijing	30	1	2017-10-01 17:05:45	2	22	22
10002	2017-10-02	Shanghai	20	1	2017-10-02 12:59:12	200	5	5
10003	2017-10-02	Guangzhou	32	0	2017-10-02 11:20:00	30	11	11
10004	2017-10-01	Shenzhen	35	0	2017-10-01 10:00:15	100	3	3
10004	2017-10-03	Shenzhen	35	0	2017-10-03 11:22:00	55	19	6
10005	2017-10-03	Changsha	29	1	2017-10-03 18:11:02	3	1	1

As you can see, the existing data and the newly imported data of User 10004 have been aggregated. Meanwhile, the new data of User 10005 have been added.

In Doris, data aggregation happens in the following 3 stages:

1. The ETL stage of each batch of import data. At this stage, the batch of import data will be aggregated internally.
2. The data compaction stage of the underlying BE. At this stage, BE will aggregate data from different batches that have been imported.
3. The data query stage. The data involved in the query will be aggregated accordingly.

At different stages, data will be aggregated to varying degrees. For example, when a batch of data is just imported, it may not be aggregated with the existing data. But for users, they can only query aggregated data. That is, what users see are the aggregated data, and they should not assume that what they have seen are not or partly aggregated. (See the [Limitations of Aggregate Model](#Limitations of Aggregate Model) section for more details.)

agg_state

AGG_STATE cannot be used as a key column, and when creating a table, you need to declare the signature of the aggregation function. Users do not need to specify a length or default value. The actual storage size of the data depends on the function implementation.

CREATE TABLE

set enable_agg_state=true;
create table aggstate(
    k1 int null,
    k2 agg_state sum(int),
    k3 agg_state group_concat(string)
)
aggregate key (k1)
distributed BY hash(k1) buckets 3
properties("replication_num" = "1");

"agg_state" is used to declare the data type as "agg_state," and "sum/group_concat" are the signatures of aggregation functions.

Please note that "agg_state" is a data type, similar to "int," "array," or "string."

"agg_state" can only be used in conjunction with the state/merge/union function combinators.

"agg_state" represents an intermediate result of an aggregation function. For example, with the aggregation function "sum," "agg_state" can represent the intermediate state of summing values like sum(1, 2, 3, 4, 5), rather than the final result.

The "agg_state" type needs to be generated using the "state" function. For the current table, it would be "sum_state" and "group_concat_state" for the "sum" and "group_concat" aggregation functions, respectively.

insert into aggstate values(1,sum_state(1),group_concat_state('a'));
insert into aggstate values(1,sum_state(2),group_concat_state('b'));
insert into aggstate values(1,sum_state(3),group_concat_state('c'));

At this point, the table contains only one row. Please note that the table below is for illustrative purposes and cannot be selected/displayed directly:

k1	k2	k3
1	sum(1,2,3)	group_concat_state(a,b,c)

Insert another record.

insert into aggstate values(2,sum_state(4),group_concat_state('d'));

The table's structure at this moment is...

k1	k2	k3
1	sum(1,2,3)	group_concat_state(a,b,c)
2	sum(4)	group_concat_state(d)

We can use the 'merge' operation to combine multiple states and return the final result calculated by the aggregation function.

mysql> select sum_merge(k2) from aggstate;
+---------------+
| sum_merge(k2) |
+---------------+
|            10 |
+---------------+

sum_merge will first combine sum(1,2,3) and sum(4) into sum(1,2,3,4), and return the calculated result. Because group_concat has a specific order requirement, the result is not stable.

mysql> select group_concat_merge(k3) from aggstate;
+------------------------+
| group_concat_merge(k3) |
+------------------------+
| c,b,a,d                |
+------------------------+

If you do not want the final aggregation result, you can use 'union' to combine multiple intermediate aggregation results and generate a new intermediate result.

insert into aggstate select 3,sum_union(k2),group_concat_union(k3) from aggstate ;

The table's structure at this moment is...

k1	k2	k3
1	sum(1,2,3)	group_concat_state(a,b,c)
2	sum(4)	group_concat_state(d)
3	sum(1,2,3,4)	group_concat_state(a,b,c,d)

You can achieve this through a query.

mysql> select sum_merge(k2) , group_concat_merge(k3)from aggstate;
+---------------+------------------------+
| sum_merge(k2) | group_concat_merge(k3) |
+---------------+------------------------+
|            20 | c,b,a,d,c,b,a,d        |
+---------------+------------------------+

mysql> select sum_merge(k2) , group_concat_merge(k3)from aggstate where k1 != 2;
+---------------+------------------------+
| sum_merge(k2) | group_concat_merge(k3) |
+---------------+------------------------+
|            16 | c,b,a,d,c,b,a          |
+---------------+------------------------+

Users can perform more detailed aggregation function operations using agg_state.

Please note that agg_state comes with a certain performance overhead.

Unique Model

In some multidimensional analysis scenarios, users are highly concerned about how to ensure the uniqueness of the Key, that is, how to create uniqueness constraints for the Primary Key. Therefore, we introduce the Unique Model. Prior to Doris 1.2, the Unique Model was essentially a special case of the Aggregate Model and a simplified representation of table schema. The Aggregate Model is implemented by Merge on Read, so it might not deliver high performance in some aggregation queries (see the Limitations of Aggregate Model section). In Doris 1.2, we have introduced a new implementation for the Unique Model--Merge on Write, which can help achieve optimal query performance. For now, Merge on Read and Merge on Write will coexist in the Unique Model for a while, but in the future, we plan to make Merge on Write the default implementation of the Unique Model. The following will illustrate the two implementations with examples.

Merge on Read ( Same Implementation as Aggregate Model)

ColumnName	Type	IsKey	Comment
user_id	BIGINT	Yes	User ID
username	VARCHAR (50)	Yes	Username
city	VARCHAR (20)	No	User location city
age	SMALLINT	No	User age
sex	TINYINT	No	User gender
phone	LARGEINT	No	User phone number
address	VARCHAR (500)	No	User address
register_time	DATETIME	No	User registration time

This is a typical user basic information table. There is no aggregation requirement for such data. The only concern is to ensure the uniqueness of the primary key. (The primary key here is user_id + username). The CREATE TABLE statement for the above table is as follows:

CREATE TABLE IF NOT EXISTS example_db.example_tbl_unique
(
`user_id` LARGEINT NOT NULL COMMENT "User ID",
`username` VARCHAR (50) NOT NULL COMMENT "Username",
`city` VARCHAR (20) COMMENT "User location city",
`age` SMALLINT COMMENT "User age",
`sex` TINYINT COMMENT "User sex",
`phone` LARGEINT COMMENT "User phone number",
`address` VARCHAR (500) COMMENT "User address",
`register_time` DATETIME COMMENT "User registration time"
)
UNIQUE KEY (`user_id`, `username`)
DISTRIBUTED BY HASH(`user_id`) BUCKETS 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1"
);

This is the same table schema and the CREATE TABLE statement as those of the Aggregate Model:

ColumnName	Type	AggregationType	Comment
user_id	BIGINT		User ID
username	VARCHAR (50)		Username
city	VARCHAR (20)	REPLACE	User location city
age	SMALLINT	REPLACE	User age
sex	TINYINT	REPLACE	User gender
phone	LARGEINT	REPLACE	User phone number
address	VARCHAR (500)	REPLACE	User address
register_time	DATETIME	REPLACE	User registration time

CREATE TABLE IF NOT EXISTS example_db.example_tbl_agg3
(
`user_id` LARGEINT NOT NULL COMMENT "User ID",
`username` VARCHAR (50) NOT NULL COMMENT "Username",
`city` VARCHAR (20) REPLACE COMMENT "User location city",
`sex` TINYINT REPLACE COMMENT "User gender",
`phone` LARGEINT REPLACE COMMENT "User phone number",
`address` VARCHAR(500) REPLACE COMMENT "User address",
`register_time` DATETIME REPLACE COMMENT "User registration time"
)
AGGREGATE KEY(`user_id`, `username`)
DISTRIBUTED BY HASH(`user_id`) BUCKETS 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1"
);

That is to say, the Merge on Read implementation of the Unique Model is equivalent to the REPLACE aggregation type in the Aggregate Model. The internal implementation and data storage are exactly the same.

SinceVersion 1.2

Merge on Write

The Merge on Write implementation of the Unique Model is completely different from that of the Aggregate Model. It can deliver better performance in aggregation queries with primary key limitations.

In Doris 1.2.0, as a new feature, Merge on Write is disabled by default, and users can enable it by adding the following property:

"enable_unique_key_merge_on_write" = "true"

NOTE:

1. It is recommended to use version 1.2.4 or above, as this version has fixed some bugs and stability issues.
2. Add the configuration item "disable_storage_page_cache=false" to the be.conf file. Failure to add this configuration item may have a significant impact on data load performance.

Take the previous table as an example, the corresponding to CREATE TABLE statement should be:

CREATE TABLE IF NOT EXISTS example_db.example_tbl_unique_merge_on_write
(
`user_id` LARGEINT NOT NULL COMMENT "User ID",
`username` VARCHAR (50) NOT NULL COMMENT "Username",
`city` VARCHAR (20) COMMENT "User location city",
`age` SMALLINT COMMENT "Userage",
`sex` TINYINT COMMENT "User gender",
`phone` LARGEINT COMMENT "User phone number",
`address` VARCHAR (500) COMMENT "User address",
`register_time` DATETIME COMMENT "User registration time"
)
UNIQUE KEY (`user_id`, `username`)
DISTRIBUTED BY HASH(`user_id`) BUCKETS 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1"
"enable_unique_key_merge_on_write" = "true"
);

The table schema produced by the above statement will be different from that of the Aggregate Model.

ColumnName	Type	AggregationType	Comment
user_id	BIGINT		User ID
username	VARCHAR (50)		Username
city	VARCHAR (20)	NONE	User location city
age	SMALLINT	NONE	User age
sex	TINYINT	NONE	User gender
phone	LARGEINT	NONE	User phone number
address	VARCHAR (500)	NONE	User address
register_time	DATETIME	NONE	User registration time

On a Unique table with the Merge on Write option enabled, during the import stage, the data that are to be overwritten and updated will be marked for deletion, and new data will be written in. When querying, all data marked for deletion will be filtered out at the file level, and only the latest data would be readed. This eliminates the data aggregation cost while reading, and supports many types of predicate pushdown now. Therefore, it can largely improve performance in many scenarios, especially in aggregation queries.

[NOTE]

1. The new Merge on Write implementation is disabled by default, and can only be enabled by specifying a property when creating a new table.
2. The old Merge on Read cannot be seamlessly upgraded to the new implementation (since they have completely different data organization). If you want to switch to the Merge on Write implementation, you need to manually execute insert into unique-mow- table select * from source table to load data to new table.
3. The two unique features delete sign and sequence col of the Unique Model can be used as normal in the new implementation, and their usage remains unchanged.

Duplicate Model

In some multidimensional analysis scenarios, there is no need for primary keys or data aggregation. For these cases, we introduce the Duplicate Model to. Here is an example:

ColumnName	Type	SortKey	Comment
timstamp	DATETIME	Yes	Log time
type	INT	Yes	Log type
error_code	INT	Yes	Error code
Error_msg	VARCHAR (1024)	No	Error details
op_id	BIGINT	No	Operator ID
op_time	DATETIME	No	Operation time

The corresponding to CREATE TABLE statement is as follows:

CREATE TABLE IF NOT EXISTS example_db.example_tbl_duplicate
(
    `timestamp` DATETIME NOT NULL COMMENT "Log time",
    `type` INT NOT NULL COMMENT "Log type",
    `error_code` INT COMMENT "Error code",
    `error_msg` VARCHAR(1024) COMMENT "Error details",
    `op_id` BIGINT COMMENT "Operator ID",
    `op_time` DATETIME COMMENT "Operation time"
)
DUPLICATE KEY(`timestamp`, `type`, `error_code`)
DISTRIBUTED BY HASH(`type`) BUCKETS 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1"
);

Different from the Aggregate and Unique Models, the Duplicate Model stores the data as they are and executes no aggregation. Even if there are two identical rows of data, they will both be retained. The DUPLICATE KEY in the CREATE TABLE statement is only used to specify based on which columns the data are sorted. (A more appropriate name than DUPLICATE KEY would be SORTING COLUMN, but it is named as such to specify the data model used. For more information, see Prefix Index.) For the choice of DUPLICATE KEY, we recommend the first 2-4 columns.

The Duplicate Model is suitable for storing raw data without aggregation requirements or primary key uniqueness constraints. For more usage scenarios, see the Limitations of Aggregate Model section.

Duplicate Model Without SORTING COLUMN (Since Doris 2.0)

When creating a table without specifying Unique, Aggregate, or Duplicate, a table with a Duplicate model will be created by default, and the SORTING COLUMN will be automatically specified.

When users do not need SORTING COLUMN or Prefix Index, they can configure the following table property:

"enable_duplicate_without_keys_by_default" = "true"

Then, when creating the default model, the sorting column will no longer be specified, and no prefix index will be created for the table to reduce additional overhead in importing and storing.

The corresponding to CREATE TABLE statement is as follows:

CREATE TABLE IF NOT EXISTS example_db.example_tbl_duplicate_without_keys_by_default
(
    `timestamp` DATETIME NOT NULL COMMENT "日志时间",
    `type` INT NOT NULL COMMENT "日志类型",
    `error_code` INT COMMENT "错误码",
    `error_msg` VARCHAR(1024) COMMENT "错误详细信息",
    `op_id` BIGINT COMMENT "负责人id",
    `op_time` DATETIME COMMENT "处理时间"
)
DISTRIBUTED BY HASH(`type`) BUCKETS 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1",
"enable_duplicate_without_keys_by_default" = "true"
);

MySQL > desc example_tbl_duplicate_without_keys_by_default;
+------------+---------------+------+-------+---------+-------+
| Field      | Type          | Null | Key   | Default | Extra |
+------------+---------------+------+-------+---------+-------+
| timestamp  | DATETIME      | No   | false | NULL    | NONE  |
| type       | INT           | No   | false | NULL    | NONE  |
| error_code | INT           | Yes  | false | NULL    | NONE  |
| error_msg  | VARCHAR(1024) | Yes  | false | NULL    | NONE  |
| op_id      | BIGINT        | Yes  | false | NULL    | NONE  |
| op_time    | DATETIME      | Yes  | false | NULL    | NONE  |
+------------+---------------+------+-------+---------+-------+
6 rows in set (0.01 sec)

Limitations of Aggregate Model

This section is about the limitations of the Aggregate Model.

The Aggregate Model only presents the aggregated data. That means we have to ensure the presentation consistency of data that has not yet been aggregated (for example, two different import batches). The following provides further explanation with examples.

Suppose that you have the following table schema:

ColumnName	Type	AggregationType	Comment
user_id	LARGEINT		User ID
date	DATE		Date when the data are imported
cost	BIGINT	SUM	Total user consumption

Assume that there are two batches of data that have been imported into the storage engine as follows:

batch 1

user_id	date	cost
10001	2017-11-20	50
10002	2017-11-21	39

batch 2

user_id	date	cost
10001	2017-11-20	1
10001	2017-11-21	5
10003	2017-11-22	22

As you can see, data about User 10001 in these two import batches have not yet been aggregated. However, in order to ensure that users can only query the aggregated data as follows:

user_id	date	cost
10001	2017-11-20	51
10001	2017-11-21	5
10002	2017-11-21	39
10003	2017-11-22	22

We have added an aggregation operator to the query engine to ensure the presentation consistency of data.

In addition, on the aggregate column (Value), when executing aggregate class queries that are inconsistent with the aggregate type, please pay attention to the semantics. For example, in the example above, if you execute the following query:

SELECT MIN(cost) FROM table;

The result will be 5, not 1.

Meanwhile, this consistency guarantee could considerably reduce efficiency in some queries.

Take the basic count (*) query as an example:

SELECT COUNT(*) FROM table;

In other databases, such queries return results quickly. Because in actual implementation, the models can get the query result by counting rows and saving the statistics upon import, or by scanning only one certain column of data to get count value upon query, with very little overhead. But in Doris's Aggregation Model, the overhead of such queries is large.

For the previous example:

batch 1

user_id	date	cost
10001	2017-11-20	50
10002	2017-11-21	39

batch 2

user_id	date	cost
10001	2017-11-20	1
10001	2017-11-21	5
10003	2017-11-22	22

Since the final aggregation result is:

user_id	date	cost
10001	2017-11-20	51
10001	2017-11-21	5
10002	2017-11-21	39
10003	2017-11-22	22

The correct result of select count (*) from table; should be 4. But if the model only scans the user_id column and operates aggregation upon query, the final result will be 3 (10001, 10002, 10003). And if it does not operate aggregation, the final result will be 5 (a total of five rows in two batches). Apparently, both results are wrong.

In order to get the correct result, we must read both the user_id and date column, and performs aggregation when querying. That is to say, in the count (*) query, Doris must scan all AGGREGATE KEY columns (in this case, user_id and date) and aggregate them to get the semantically correct results. That means if there are many aggregated columns, count (*) queries could involve scanning large amounts of data.

Therefore, if you need to perform frequent count (*) queries, we recommend that you simulate count (*) by adding a column of value 1 and aggregation type SUM. In this way, the table schema in the previous example will be modified as follows:

ColumnName	Type	AggregationType	Comment
user ID	BIGINT		User ID
date	DATE		Date when the data are imported
Cost	BIGINT	SUM	Total user consumption
count	BIGINT	SUM	For count queries

The above adds a count column, the value of which will always be 1, so the result of select count (*) from table; is equivalent to that of select sum (count) from table; The latter is much more efficient than the former. However, this method has its shortcomings, too. That is, it requires that users will not import rows with the same values in the AGGREGATE KEY columns. Otherwise, select sum (count) from table; can only express the number of rows of the originally imported data, instead of the semantics of select count (*) from table;

Another method is to add a cound column of value 1 but aggregation type of REPLACE. Then select sum (count) from table; and select count (*) from table; could produce the same results. Moreover, this method does not require the absence of same AGGREGATE KEY columns in the import data.

Merge on Write of Unique Model

The Merge on Write implementation in the Unique Model does not impose the same limitation as the Aggregate Model. In Merge on Write, the model adds a delete bitmap for each imported rowset to mark the data being overwritten or deleted. With the previous example, after Batch 1 is imported, the data status will be as follows:

batch 1

user_id	date	cost	delete bit
10001	2017-11-20	50	false
10002	2017-11-21	39	false

After Batch 2 is imported, the duplicate rows in the first batch will be marked as deleted, and the status of the two batches of data is as follows

batch 1

user_id	date	cost	delete bit
10001	2017-11-20	50	true
10002	2017-11-21	39	false

batch 2

user_id	date	cost	delete bit
10001	2017-11-20	1	false
10001	2017-11-21	5	false
10003	2017-11-22	22	false

In queries, all data marked true in the delete bitmap will not be read, so there is no need for data aggregation. Since there are 4 valid rows in the above data, the query result should also be 4. This also enables minimum overhead since it only scans one column of data.

In the test environment, count(*) queries in Merge on Write of the Unique Model deliver 10 times higher performance than that of the Aggregate Model.

Duplicate Model

The Duplicate Model does not impose the same limitation as the Aggregate Model because it does not involve aggregation semantics. For any columns, it can return the semantically correct results in count (*) queries.

Key Columns

For the Duplicate, Aggregate, and Unique Models, the Key columns will be specified when the table is created, but there exist some differences: In the Duplicate Model, the Key columns of the table can be regarded as just "sorting columns", but not unique identifiers. In Aggregate and Unique Models, the Key columns are both "sorting columns" and "unique identifier columns".

Suggestions for Choosing Data Model

Since the data model was established when the table was built, and irrevocable thereafter, it is very important to select the appropriate data model.

1. The Aggregate Model can greatly reduce the amount of data scanned and query computation by pre-aggregation. Thus, it is very suitable for report query scenarios with fixed patterns. But this model is unfriendly to count (*) queries. Meanwhile, since the aggregation method on the Value column is fixed, semantic correctness should be considered in other types of aggregation queries.
2. The Unique Model guarantees the uniqueness of primary key for scenarios requiring a unique primary key. The downside is that it cannot exploit the advantage brought by pre-aggregation such as ROLLUP in queries. Users who have high-performance requirements for aggregate queries are recommended to use the newly added Merge on Write implementation since version 1.2.
3. The Duplicate Model is suitable for ad-hoc queries of any dimensions. Although it may not be able to take advantage of the pre-aggregation feature, it is not limited by what constrains the Aggregate Model and can give full play to the advantage of columnar storage (reading only the relevant columns, but not all Key columns).
4. If user need to use partial-update, please refer to documnet partial-update