Make plan cache compatible with stale read and history read

[djshow832]

Background

This is a subtask of #21094.

The prepared plan cache caches the physical plan for each cache key.

The cache key depends on:

• The prepared statement ID
• The connection ID and current database name
• The table schema, a.k.a. the schema version
• The snapshot timestamp, if it's a history read
• Other variables or configurations, such as SQL mode, timezone

Among these variables, the schema version and snapshot timestamp of stale reads need to be handled carefully.

Solution

Let's discuss these two variables separately.

Snapshot timestamp

Actually, the snapshot timestamp doesn't affect the physical plan, so it can be removed. I have submitted issue #22497.

For now, we just ignore this variable and keep it untouched.

Schema version

schemaVersion in the cache key is the schema version when the PREPARE statement is parsed, rather than the schema version when the EXECUTE statement is executed or the schema version recorded in the prepared AST.

When querying the plan cache, callers construct a cache key with the schema version recorded in the prepared AST. After schema changes, the schema version in the prepared AST may also change, so the plans built with older schema versions won't be applied.

When executing the EXECUTE statement, the executor ensures that it always uses the schema version with which the transaction begins. However, this schema version may be too new for stale reads. The easiest solution is forbidding any related schema changes between the transaction start ts and the time when the transaction really begins. This solution is described in #22427.

To summarize, the schemaVersion can also stay what it is for stale reads.

[Yisaer]

/assign

[Yisaer]

Currently we forbid stale read with plan cache.