This removes a whole set of GUCs which are no longer used in the
codebase. Also remove relevant documentation.
Reviewed-by: NAshwin Agrawal <aagrawal@pivotal.io>
Reviewed-by: NHeikki Linnakangas <hlinnakangas@pivotal.io>
Reviewed-by: NGeorgios Kokolatos <gkokolatos@pivotal.io>

This re-implements compression that was lost in the WorkFile/BufFile
refactoring. This new implementation uses Zstandard rather than zlib.

Compression can be enabled with "gp_workfile_compression=on". It is
disabled by default.
Reviewed-by: NMel Kiyama <mkiyama@pivotal.io>
Reviewed-by: NYandong Yao <yyao@pivotal.io>

Temporary files have been somewhat inconsistent across different
operations. Some operations used the Greenplum-specific "workfile" API,
while others used the upstream BufFile API directly.

The workfile API provides some extra features: workfiles are visible in
the gp_toolkit views, and you can limit their size with the
gp_workfile_limit_* GUCs. The temporary files that didn't go through the
workfile API were exempt, which is not cool.

To make things consistent, remove the workfile APIs. Use BufFiles
directly everywhere. Re-implement the user-facing view and tracking the
limits, on top of the BufFile API, so that those features are not lost.

The workfile API also supported compressing the temporary files using
zlib. That feature is lost with this commit, but will be re-introduced by
the next commit.

Another feature that this removes, is checksumming temporary files.
That doesn't seem very useful, so we can probably live without it. But
if it's still needed, then that should also be re-implemented on top
of the BufFile API later.

Discussion: https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/8Xe9MGor0pM/SjqiOo83BAAJReviewed-by: NMel Kiyama <mkiyama@pivotal.io>
Reviewed-by: NYandong Yao <yyao@pivotal.io>

* docs - remove track_count GUC

* docs - removed GUC track_activities

will be backported to 5X_STABLE

The support for sending alerts via Email or SNMP was quite a kludge,
and there are much better external tools for managing alerts than
what we can supply in core anyways so this retires the capability.

All references to alert sending in the docs are removed, but there
needs to be section written about how to migrate off this feature
in the release notes or a similar location.

Discussion: https://github.com/greenplum-db/gpdb/pull/6384

Given the online gpexpand work, the gp_num_contents_in_cluster GUC is
unused. So, delete the same from code to avoid confusions and eliminate this
long argument required to start a postgres instance in gpdb.

Now there is only the async dispatcher.  The dispatcher API interface is
kept so we might add new backend in the future.

The GUC gp_connections_per_thread is also retired which was used to
switch between the async and threaded backends.

This GUC is now unused after the major COPY refactor.
Co-authored-by: NChris Hajas <chajas@pivotal.io>
Co-authored-by: NKevin Yeap <kyeap@pivotal.io>

Instead of completely disabling the generation of Paths with disabled
plan types, add a high penalty to their cost estimates, like in the
upstream. This reduces our diff vs. upstream, making future merges more
straightforward.

Discussion: https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/Az2cDcqf73g/_tY6Yv1kBgAJCo-authored-by: NDaniel Gustafsson <dgustafsson@pivotal.io>
Reviewed-by: NVenkatesh Raghavan <vraghavan@pivotal.io>
Reviewed-by: NRichard Guo <riguo@pivotal.io>

Since persistent tables were removed, these were not used for anything.

Plenty remains, but this is a start.

* De-conflate gpperfmon & gpcc. Conditionalize gpcc. Remove unreferenced topics.

* Update GPAdminGuide.ditaval

Remove addition to Admin Guide ditaval

* Fixes from review.

* Relocates gpperfmon reference from GPCC

* Remove emcconnect history and itorators from PDF ditamap

* Remove iterator metrics subtopic from PDF ditamap

* Additional edits to acknowledge gpperfmon without GPCC

* Incorporate gpperfmon.conf reference into gpperfmon_install reference

The gp_external_grant_privileges GUC was needed before 4.0 to let
non superusers create external tables for gphdfs and http protocols.
This GUC was however deprecated during the 4.3 cycle so remove all
traces of it. The utility of the GUC was replaced in 4.0 when rights
management for external tables was implemented with the normal GRANT
REVOKE framework so this has been dead code for quite some time.

Remove GUC, code which handles it, all references to it from the
documentation and a release notes entry.