AliSQL: [Feature] Issue#43 Add Column Dynamically

Description:

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The original adding column will move the data. it will consume long time.

Now the new row_format COMFORT for InnoDB can add column dynamically.

The innodb comfort record format such as:

  [lens | n_nulls | n_fields | extra | id...]

    1. extra info_bits will occupy 1 bit to flag comfort.

    2. n_fields will occupy 1 or 2 bytes to save the column number.

Then add column action will only change the dictionary and cache.

Limition:

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The column only can added at last, and nullable, non-default value.

Usage:

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create table t (id int) engine=innodb row_format=comfort.

alter table t add col1 int;