Re: Prepared statements and generic plans

> I'd maybe go with something like this:
>
> All executions of a prepared statement having zero parameters will use the same plan so the planning
> time taken during the first execution will be spread across all subsequent executions.  For statements
> having parameters the first five executions will result in value-specific plans as previously
> described.  However, on the sixth execution a generic plan will also be computed and if the average
> planning + execution cost of all previous value-specific plans is about equal to the execution cost of
> the generic plan the generic plan will be chosen for that and all subsequent executions.

I think that is much better, but I suggest this wording:

"All executions of a prepared statement having zero parameters use the same plan, so they
 will use the generic plan immediately.  For statements having parameters the first five executions
 will result in value-specific plans as previously described.
 However, on the sixth execution a generic plan will also be computed, and if the average cost estimate
 of all previous value-specific plans is about equal to the cost estimate of the generic plan,
 the generic plan will be chosen for that and all subsequent executions."

This emphasizes that it is only estimates we are dealing with, otherwise it would be hard
to understand why estimation errors can lead to generic plans being chosen that are much worse.

> <existing next paragraph>
>
> If we are getting generic plans significantly cheaper than the value-specific plans I suspect there is
> a problem...so any comparison that indicates "less-than" is prone to cause confusion.  The original is
> worded well on this point: "...generic plan appears to be not much more expensive..." but lacks detail
> elsewhere.

I don't quite get that.  Do you mean the same thing that I wrote above?

> This part:
>
> !    A generic plan assumes each value supplied to <command>EXECUTE</command>
> !    is one of the column's distinct values and that column values are
> !    uniformly distributed.  For example, if statistics records three
> !    distinct column values, a generic plan assumes a column equality
> !    comparison will match 33% of processed rows.  Column statistics
> !    also allows generic plans to accurately compute the selectivity of
> !    unique columns.  Comparisons on non-uniformly-distributed columns and
> !    specification of non-existent values affects the average plan cost,
> !    and hence if and when a generic plan is chosen.  [elided the last sentence, placed in the first
> paragraph]
>
> I'm not sure of the specific goal here but this level detail seems a bit out-of-place in the SQL
> Command documentation.  So, do we want this user-facing and if so do we want it here?

[...]

> This leaves Bruce's second alteration: which probably should follow the rest over to chapter 66.  The
> point of the existing sentence is to give the casual user the means to detect the current type of plan
> and I think that is all that is needed here.

I agree that this is too much detail.
I would vote for omitting it altogether.

Anybody who needs that level of detail is better served with the source anyway.