[TTL] Historical data may never expire

[cangfengzhs]

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Describe the bug (required)

create tag ttl_tag1(age int);
insert vertex ttl_tag1(age) VALUES "1":(10),"2":(20),"3":(30);
alter tag ttl_tag1 add (ttl timestamp default now());
alter tag ttl_tag1 ttl_duration=10, ttl_col="ttl";
insert vertex ttl_tag1(age) VALUES "4":(40);
match (v:ttl_tag1) return v limit 10;

After the above series of operations. The ttl_tag1 of "1", "2" and "3" will never expire.

This is because there is a problem with our definition of default. In this case, the default value of ttl is now(). now(), which will be stored in the definition of ttl (it is the function now(), not the return value of it).

When we read a ttl if the property does not exist in tag1.ttl, it will get the default value from the schema definition. So for ttl_tag1 of "1", "2" and "3", the latest now() will be calculated every time when reading, so they will never expire.

Your Environments (required)

• OS: uname -a
• Compiler: g++ --version or clang++ --version
• CPU: lscpu
• Commit id (e.g. a3ffc7d8)

How To Reproduce(required)

Steps to reproduce the behavior:

1. Step 1
2. Step 2
3. Step 3

Expected behavior

Additional context

[cangfengzhs]

Here is a relatively reasonable repair plan. We define two default values for each property, default_read_value and default_write_value.

When we define a DEFAULT, we assign it to both default_read_value and default_write_value. However, the difference is that default_read_value must be a constant, and default_write_value is the actually defined DEFAULT.

In the above case, default_read_value stores the actual time (the return value of now()) when ttl is defined, and default_write_value stores the now() function itself.

Later, in the process of using ttl, if it is found that there is no ttl attribute when reading its value, it will be filled with default_read_value. If no ttl value is given when writing, it will be filled with default_write_value.

In addition, subsequent modifications to 'DEFAULT' will only modify the default_write_value, and default_read_value will never modify

[cangfengzhs]

And, if the value of property ttlis NULL, tag also never expire.

[cangfengzhs]

Another relatively simple repair method is to prohibit the use of "now()" as the default value on the ttl column, and do not allow the value to be NULL

[cangfengzhs]

@critical27 @yixinglu

[cangfengzhs]

@MuYiYong

[critical27]

I suggest forbid use a default value of now, in alter tag ttl_tag1 add (ttl timestamp default now());

[HarrisChu]

prefer to mark as a known issue.
and we could re-design in v5.0

[Sophie-Xie]

@abby-cyber discussed it with @MuYiYong offline: he suggested to update doc.

[abby-cyber]

https://github.com/vesoft-inc/nebula-docs-cn/pull/2173/files