workload: reintroduce old prepare behavior

[rafiss]

This reverts commit f446498,
and makes the 'prepare' setting work more closely to how it used to.

This is needed because we are seeing that the schemachange workload is
deallocating huge amounts of automatically prepared statements. By
explicitly preparing statements again, they should no longer be
deallocated.

This also disables the automatic prepared statement cache in pgx entirely,
which was enabled in jackc/pgx@0c3e59b

Release justification: test-only change
Release note: None

[cockroach-teamcity]

This change is 

[rafiss]

I ran schemachange/index/tpcc/w=1000 with this change and still see hundreds/thousands of DEALLOCATE statements.

[rafiss]

Ah I see, jackc/pgx@0c3e59b is new in pgx4 and enables this automatic prepared statement caching. I made a 2nd commit in this PR which turns that off and it has gotten rid of all the DEALLOCATE statements.

[ajwerner]

though I wonder how important the first commit is. What would happen if we only merged the second commit?

Reviewed 12 of 12 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @jordanlewis)

[rafiss]

I agree, the 2nd commit is what prevents the thousands of DEALLOCATE statements.

I think the 1st commit is still good since we shouldn't change the client behavior unintentionally. If we didn't merge the first commit, what would happen is that every query would be run using Parse/Bind/Execute every time, using an anonymous prepared statement. I feel that if we want that behavior it's better to opt-in explicitly by using the noprepare workload setting.

[rafiss]

bors r=ajwerner

[craig]

Build succeeded:

• GitHub CI (Cockroach)