Re: track generic and custom plans in pg_stat_statements

> > It may be more efficient to set the is_generic_plan option at the top
> > plan node (PlannedStmt) and reference it wherever necessary. To identify
> > a cached plan, we may consider pushing the CachedPlan/CachedPlanSource
> > pointer down throughout pg_plan_query and maintaining a reference to the
> > plan (or simply setting a boolean flag) at the same location — inside
> > the PlannedStmt.
>
> We will need a field to store an enum. let's call it CachedPlanType
> with the types of cached plan. We need to be able to differentiate
> when cached plans are not used, so a simple boolean is not
> sufficient.
> ```
> typedef enum CachedPlanType
> {
> PLAN_CACHE_NOT_SET, /* Not a cached plan */
> PLAN_CACHE_GENERIC, /* Generic cached plan */
> PLAN_CACHE_CUSTOM, /* Custom cached plan */
> } CachedPlanType;
> ```
> We can track a field for this enum in PlannedStmt and initialize
> it to PLAN_CACHE_NOT_SET whenever we makeNode(PlannedStmt).
> In GetCachedPlan, we iterate through plan->stmt_list to set the
> value for the PlannedStmt.
>
> CachedPlanType will have to be defined in nodes.h for this to work.
>
> We can avoid the struct altogether and define the new PlannedStmt
> field as an "int" and bit-pack the types.
>
> I prefer the CachedPlanType struct to do this, as it's cleaner and
> self-documenting.

v13 is the implementation using PlannedStmt as described above.

--
Sami