refresh_by_match_merge() has some issues in the way it builds a SQL
query to construct the "diff" table:

1. It doesn't require the selected unique index(es) to be indimmediate.
2. It doesn't pay attention to the particular equality semantics enforced
by a given index, but just assumes that they must be those of the column
datatype's default btree opclass.
3. It doesn't check that the indexes are btrees.
4. It's insufficiently careful to ensure that the parser will pick the
intended operator when parsing the query.  (This would have been a
security bug before CVE-2018-1058.)
5. It's not careful about indexes on system columns.

The way to fix #4 is to make use of the existing code in ri_triggers.c
for generating an arbitrary binary operator clause.  I chose to move
that to ruleutils.c, since that seems a more reasonable place to be
exporting such functionality from than ri_triggers.c.

While #1, #3, and #5 are just latent given existing feature restrictions,
and #2 doesn't arise in the core system for lack of alternate opclasses
with different equality behaviors, #4 seems like an issue worth
back-patching.  That's the bulk of the change anyway, so just back-patch
the whole thing to 9.4 where this code was introduced.

Discussion: https://postgr.es/m/[email protected]

This is the commit message #2:

Tracking on which foreign servers we already started COPY.

If foreign server holds several partitions, COPY FROM to local root partition
will try to perform several copies at the same time through one connection,
obviously without much success. Now we track that and start/end COPY only once.

We also allow to pass destination relation name which may be different from
foreing table -- so we can copy into foreign root partition in shardman. This
is pretty narrow solution. However, keeping several connections to the same
foreign server requires significant changes, especially in 2pc handling, so
staying here for now.

This is the commit message #3:

Allow COPY FROM to par8d table even if some FDW parts can't do that.

This behaviour was broken in patches allowing COPY FROM to FDW tables.

This is the commit message #4:

COPY FROM deparse more complete, PG_SHARDMAN macro.

Now column names, FORCE NULL and FORCE NOT NULL are deparsed too.
PG_SHARDMAN macro ensures this PG contains patches for Postgres.

This is the commit message #5:

Disable COPY FROM to foreign parts, because no generic impl exists.

This is the commit message #6:

Fix COPY FROM deparse, forgotten comma for FORCE_NULL etc.

Cherry-picked from af234df.

This is the commit message #2:

Tracking on which foreign servers we already started COPY.

If foreign server holds several partitions, COPY FROM to local root partition
will try to perform several copies at the same time through one connection,
obviously without much success. Now we track that and start/end COPY only once.

We also allow to pass destination relation name which may be different from
foreing table -- so we can copy into foreign root partition in shardman. This
is pretty narrow solution. However, keeping several connections to the same
foreign server requires significant changes, especially in 2pc handling, so
staying here for now.

This is the commit message #3:

Allow COPY FROM to par8d table even if some FDW parts can't do that.

This behaviour was broken in patches allowing COPY FROM to FDW tables.

This is the commit message #4:

COPY FROM deparse more complete, PG_SHARDMAN macro.

Now column names, FORCE NULL and FORCE NOT NULL are deparsed too.
PG_SHARDMAN macro ensures this PG contains patches for Postgres.

This is the commit message #5:

Disable COPY FROM to foreign parts, because no generic impl exists.

This is the commit message #6:

Fix COPY FROM deparse, forgotten comma for FORCE_NULL etc.

Cherry-picked from af234df.

This is the commit message #2:

Tracking on which foreign servers we already started COPY.

If foreign server holds several partitions, COPY FROM to local root partition
will try to perform several copies at the same time through one connection,
obviously without much success. Now we track that and start/end COPY only once.

We also allow to pass destination relation name which may be different from
foreing table -- so we can copy into foreign root partition in shardman. This
is pretty narrow solution. However, keeping several connections to the same
foreign server requires significant changes, especially in 2pc handling, so
staying here for now.

This is the commit message #3:

Allow COPY FROM to par8d table even if some FDW parts can't do that.

This behaviour was broken in patches allowing COPY FROM to FDW tables.

This is the commit message #4:

COPY FROM deparse more complete, PG_SHARDMAN macro.

Now column names, FORCE NULL and FORCE NOT NULL are deparsed too.
PG_SHARDMAN macro ensures this PG contains patches for Postgres.

This is the commit message #5:

Disable COPY FROM to foreign parts, because no generic impl exists.

This is the commit message #6:

Fix COPY FROM deparse, forgotten comma for FORCE_NULL etc.

Cherry-picked from af234df.

This is the commit message #2:

Tracking on which foreign servers we already started COPY.

If foreign server holds several partitions, COPY FROM to local root partition
will try to perform several copies at the same time through one connection,
obviously without much success. Now we track that and start/end COPY only once.

We also allow to pass destination relation name which may be different from
foreing table -- so we can copy into foreign root partition in shardman. This
is pretty narrow solution. However, keeping several connections to the same
foreign server requires significant changes, especially in 2pc handling, so
staying here for now.

This is the commit message #3:

Allow COPY FROM to par8d table even if some FDW parts can't do that.

This behaviour was broken in patches allowing COPY FROM to FDW tables.

This is the commit message #4:

COPY FROM deparse more complete, PG_SHARDMAN macro.

Now column names, FORCE NULL and FORCE NOT NULL are deparsed too.
PG_SHARDMAN macro ensures this PG contains patches for Postgres.

This is the commit message #5:

Disable COPY FROM to foreign parts, because no generic impl exists.

This is the commit message #6:

Fix COPY FROM deparse, forgotten comma for FORCE_NULL etc.