Rather than trying hard to have PostgreSQL fully qualify the index name
with tricks around search_path setting at the time ::regclass is
executed, simply join on pg_namespace to retrieve that schema in a new
slot in our pgsql-index structure so that we can then reuse it when
needed.
Also add a test case for the scenario, including both a UNIQUE
constraint and a classic index, because the DROP and CREATE/ALTER
instructions differ.
The new option 'drop indexes' reuses the existing code to build all the
indexes in parallel but failed to properly account for that fact in the
summary report with timings.
While fixing this, also fix the SQL used to re-establish the indexes and
associated constraints to allow for parallel execution, the ALTER TABLE
statements would block in ACCESS EXCLUSIVE MODE otherwise and make our
efforts vain.
When loading against a table that already has index definitions, the
load can be quite slow. Previous commit introduced a warning in such a
case. This commit introduces the option "drop indexes" that is not used
by default.
When this option is used, pgloader drops the indexes before loading the
data then create the indexes again with the same definitions as before.
All the indexes are created again in parallel to optimize performances.
Only primary key indexes can't be created in parallel, so those are
created in two steps (create unique index then alter table).
