[YSQL] Add support for query plan management

[sushantrmishra]

Jira Link: DB-800

Description

Query plan management main objectives:

• Preventing plan regressions when the database system changes
• Controlling when the query optimizer can use new plans

Using the query plan management allow control execution plans for a set of statements. You can do the following:

• Improve plan stability by forcing the optimizer to choose from a small number of known, good plans.
• Optimize plans centrally and then distribute the best plans globally.
• Try new optimizer features with less risk, because you can choose to approve only the plan changes that improve performance.

Some open source option to consider.

• pg_store_plans
• pg_plan_guarantee

[mtakahar]

pg_store_plans

• This is a plan history tracking feature rather than a plan stability tool.
• This is extremely useful to detect any plan changes and how different plans have performed.
• We should have something like this as a part of the observability project.

pg_plan_guarantee

• This tool just prints the plan data structure as a serialized text then the user has to specify that plan text along with the query.
• Setting aside the poor user interface, saving the raw plan representation is dangerous and will easily break with some schema changes and after an upgrade (especially when we upgrade the postgres, but also possible when some push down feature additions and behavior changes.) The plan representation must not contain any column/table positions or Oids of any kind.

A potential and more intuitive way of fixing the plan for each given query would be to generate and store a set of hints in the hint_plan.hints table.

• The pg_hints feature may need to be improved to work more reliably or extended to allow more thorough controls, however, the effort will also help regular hint usages, too.
• In the future we can also use the hints by an automated query tuning tool to either completely or specify only the key aspects of the plan, generate the "Rows" hint to compensate the estimation errors, etc., too.