Maksim Myskov created IGNITE-21384:
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Summary: Initialize all rows with new column's default value
Key: IGNITE-21384
URL: https://issues.apache.org/jira/browse/IGNITE-21384
Project: Ignite
Issue Type: New Feature
Reporter: Maksim Myskov
In case of adding a new column with a default value specified (for example:
ALTER TABLE ADD COLUMN col INT DEFAULT 10), all existing rows (that were added
before "alter table") must be initialized with a new value.
h3. Current behaviour
The default value of a column is evaluated on read.
h3. Motivation
This is a pre-requisite for adding arbitrary expressions as default values.
Let's imagine a simple case:
{code:sql}
ALTER TABLE ADD COLUMN modified TIMESTAMP DEFAULT CURRENT_TIMESTAMP
{code}
With the current behavior for all existing rows the value of "modified" column
will be evaluated on every read.
Many DBs use the proposed behavior:
* PostgreSQL
* CockroachDB
* Yugabyte
For example, from PostgreSQL
[docs|https://www.postgresql.org/docs/current/sql-altertable.html#SQL-ALTERTABLE-NOTES]:
{noformat}
Adding a column with a volatile DEFAULT or changing the type of an existing
column will require the entire table and its indexes to be rewritten.
{noformat}
h3. Downsides
Adding a new column with a default value will be a much more expensive
operation and can take a significant time on large tables
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