Add the ability to create indexes 'concurrently', that is, without

blocking concurrent writes to the table.  Greg Stark, with a little help
from Tom Lane.

src/test/regress/sql/create_index.sql

@@ -219,3 +219,43 @@ INSERT INTO func_index_heap VALUES('QWERTY');
 create unique index hash_f8_index_1 on hash_f8_heap(abs(random));
 create unique index hash_f8_index_2 on hash_f8_heap((seqno + 1), random);
 create unique index hash_f8_index_3 on hash_f8_heap(random) where seqno > 1000;
+
+--
+-- Try some concurrent index builds
+--
+-- Unfortunately this only tests about half the code paths because there are
+-- no concurrent updates happening to the table at the same time.
+
+CREATE TABLE concur_heap (f1 text, f2 text);
+-- empty table
+CREATE INDEX CONCURRENTLY concur_index1 ON concur_heap(f2,f1);
+INSERT INTO concur_heap VALUES  ('a','b');
+INSERT INTO concur_heap VALUES  ('b','b');
+-- unique index
+CREATE UNIQUE INDEX CONCURRENTLY concur_index2 ON concur_heap(f1);
+-- check if constraint is set up properly to be enforced
+INSERT INTO concur_heap VALUES ('b','x');
+-- check if constraint is enforced properly at build time
+CREATE UNIQUE INDEX CONCURRENTLY concur_index3 ON concur_heap(f2);
+-- test that expression indexes and partial indexes work concurrently
+CREATE INDEX CONCURRENTLY concur_index4 on concur_heap(f2) WHERE f1='a';
+CREATE INDEX CONCURRENTLY concur_index5 on concur_heap(f2) WHERE f1='x';
+CREATE INDEX CONCURRENTLY concur_index6 on concur_heap((f2||f1));
+
+-- You can't do a concurrent index build in a transaction
+BEGIN;
+CREATE INDEX CONCURRENTLY concur_index7 ON concur_heap(f1);
+COMMIT;
+
+-- But you can do a regular index build in a transaction
+BEGIN;
+CREATE INDEX std_index on concur_heap(f2);
+COMMIT;
+
+-- check to make sure that the failed indexes were cleaned up properly and the
+-- successful indexes are created properly. Notably that they do NOT have the
+-- "invalid" flag set.
+
+\d concur_heap
+
+DROP TABLE concur_heap;