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EmmyMiao87 b3a52a05d5 [Update] Support update syntax (#6230 )

[Update] Support update syntax

    The current update syntax only supports updating the filtered data of a single table.

    Syntax:

     * UPDATE table_reference
     *     SET assignment_list
     *     [WHERE where_condition]
     *
     * value:
     *     {expr}
     *
     * assignment:
     *     col_name = value
     *
     * assignment_list:
     *     assignment [, assignment] ...

    Example
    Update unique_table
         set v1=1
         where k1=1

    New Frontend Config: enable_concurrent_update
    This configuration is used to control whether multi update stmt can be executed concurrently in one table.
    Default value is false which means A table can only have one update task being executed at the same time.
    If users want to update the same table concurrently,
      they need to modify the configuration value to true and restart the master frontend.
    Concurrent updates may cause write conflicts, the result is uncertain, please be careful.

    The main realization principle:
    1. Read the rows that meet the conditions according to the conditions set by where clause.
    2. Modify the result of the row according to the set clause.
    3. Write the modified row back to the table.

    Some restrictions on the use of update syntax.
    1. Only the unique table can be updated
    2. Only the value column of the unique table can be updated
    3. The where clause currently only supports single tables

    Possible risks:
    1. Since the current implementation method is a row update,
         when the same table is updated concurrently, there may be concurrency conflicts which may cause the incorrect result.
    2. Once the conditions of the where clause are unsatisfactory, it is likely to cause a full table scan and affect query performance.
       Please pay attention to whether the column in the where clause can match the index when using it.

    [Docs][Update] Add update document and sql-reference

    Fixed #6229

2021-07-27 13:38:15 +08:00

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title, language

title	language
UPDATE	en

UPDATE

description

Syntax

UPDATE table_name 
    SET assignment_list
    WHERE expression

value:
    {expr | DEFAULT}

assignment:
    col_name = value

assignment_list:
    assignment [, assignment] ...

Parameters

table_name: The target table of the data to be updated. Can be in the form of 'db_name.table_name'
assignment_list: The target column to be updated. Can be in the form of 'col_name = value, col_name = value'
where expression: The condition to be updated is an expression that returns true or false

Note

The current UPDATE statement only supports row updates on the Unique model, and there may be data conflicts caused by concurrent updates. Currently Doris does not deal with such problems, and users are required to avoid such problems from the business side.

example

The test table is a unique model table, which contains four columns: k1, k2, v1, v2. Among them, k1, k2 are keys, v1, v2 are values, and the aggregation method is Replace.

Update the v1 column that satisfies the conditions k1 =1 and k2 = 2 in the'test' table to 1

UPDATE test SET v1 = 1 WHERE k1=1 and k2=2;

Increment the v1 column of the column with k1=1 in the'test' table by 1

UPDATE test SET v1 = v1+1 WHERE k1=1;

keyword

UPDATE

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