Add support for INSERT ... ON CONFLICT DO NOTHING/UPDATE.

The newly added ON CONFLICT clause allows to specify an alternative to
raising a unique or exclusion constraint violation error when inserting.
ON CONFLICT refers to constraints that can either be specified using a
inference clause (by specifying the columns of a unique constraint) or
by naming a unique or exclusion constraint.  DO NOTHING avoids the
constraint violation, without touching the pre-existing row.  DO UPDATE
SET ... [WHERE ...] updates the pre-existing tuple, and has access to
both the tuple proposed for insertion and the existing tuple; the
optional WHERE clause can be used to prevent an update from being
executed.  The UPDATE SET and WHERE clauses have access to the tuple
proposed for insertion using the "magic" EXCLUDED alias, and to the
pre-existing tuple using the table name or its alias.

This feature is often referred to as upsert.

This is implemented using a new infrastructure called "speculative
insertion". It is an optimistic variant of regular insertion that first
does a pre-check for existing tuples and then attempts an insert.  If a
violating tuple was inserted concurrently, the speculatively inserted
tuple is deleted and a new attempt is made.  If the pre-check finds a
matching tuple the alternative DO NOTHING or DO UPDATE action is taken.
If the insertion succeeds without detecting a conflict, the tuple is
deemed inserted.

To handle the possible ambiguity between the excluded alias and a table
named excluded, and for convenience with long relation names, INSERT
INTO now can alias its target table.

Bumps catversion as stored rules change.

Author: Peter Geoghegan, with significant contributions from Heikki
    Linnakangas and Andres Freund. Testing infrastructure by Jeff Janes.
Reviewed-By: Heikki Linnakangas, Andres Freund, Robert Haas, Simon Riggs,
    Dean Rasheed, Stephen Frost and many others.

contrib/test_decoding/sql/ddl.sql

@@ -84,6 +84,18 @@ COMMIT;
 -- show changes
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
 
+-- ON CONFLICT DO UPDATE support
+BEGIN;
+INSERT INTO replication_example(id, somedata, somenum) SELECT i, i, i FROM generate_series(-15, 15) i
+  ON CONFLICT (id) DO UPDATE SET somenum = excluded.somenum + 1;
+COMMIT;
+
+/* display results, but hide most of the output */
+SELECT count(*), min(data), max(data)
+FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1')
+GROUP BY substring(data, 1, 40)
+ORDER BY 1,2;
+
 -- hide changes bc of oid visible in full table rewrites
 CREATE TABLE tr_unique(id2 serial unique NOT NULL, data int);
 INSERT INTO tr_unique(data) VALUES(10);
@@ -114,6 +126,16 @@ FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids',
 GROUP BY substring(data, 1, 24)
 ORDER BY 1,2;
 
+-- check that a large, spooled, upsert works
+INSERT INTO tr_etoomuch (id, data)
+SELECT g.i, -g.i FROM generate_series(8000, 12000) g(i)
+ON CONFLICT(id) DO UPDATE SET data = EXCLUDED.data;
+
+SELECT substring(data, 1, 29), count(*)
+FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1')
+GROUP BY 1
+ORDER BY min(location - '0/0');
+
 /*
  * check whether we decode subtransactions correctly in relation with each
  * other

contrib/test_decoding/sql/toast.sql

@@ -25,6 +25,11 @@ UPDATE xpto SET toasted_col1 = (SELECT string_agg(g.i::text, '') FROM generate_s
 
 UPDATE xpto SET rand1 = 123.456 WHERE id = 1;
 
+-- updating external via INSERT ... ON CONFLICT DO UPDATE
+INSERT INTO xpto(id, toasted_col2) VALUES (2, 'toasted2-upsert')
+ON CONFLICT (id)
+DO UPDATE SET toasted_col2 = EXCLUDED.toasted_col2 || xpto.toasted_col2;
+
 DELETE FROM xpto WHERE id = 1;
 
 DROP TABLE IF EXISTS toasted_key;