Add a \gexec command to psql for evaluation of computed queries.

\gexec executes the just-entered query, like \g, but instead of printing
the results it takes each field as a SQL command to send to the server.
Computing a series of queries to be executed is a fairly common thing,
but up to now you always had to resort to kluges like writing the queries
to a file and then inputting the file.  Now it can be done with no
intermediate step.

The implementation is fairly straightforward except for its interaction
with FETCH_COUNT.  ExecQueryUsingCursor isn't capable of being called
recursively, and even if it were, its need to create a transaction
block interferes unpleasantly with the desired behavior of \gexec after
a failure of a generated query (i.e., that it can continue).  Therefore,
disable use of ExecQueryUsingCursor when doing the master \gexec query.
We can still apply it to individual generated queries, however, and there
might be some value in doing so.

While testing this feature's interaction with single-step mode, I (tgl) was
led to conclude that SendQuery needs to recognize SIGINT (cancel_pressed)
as a negative response to the single-step prompt.  Perhaps that's a
back-patchable bug fix, but for now I just included it here.

Corey Huinker, reviewed by Jim Nasby, Daniel Vérité, and myself

src/test/regress/expected/psql.out

@@ -51,6 +51,51 @@ select 10 as test01, 20 as test02 from generate_series(1,3) \gset
 more than one row returned for \gset
 select 10 as test01, 20 as test02 from generate_series(1,0) \gset
 no rows returned for \gset
+\unset FETCH_COUNT
+-- \gexec
+create temporary table gexec_test(a int, b text, c date, d float);
+select format('create index on gexec_test(%I)', attname)
+from pg_attribute
+where attrelid = 'gexec_test'::regclass and attnum > 0
+order by attnum
+\gexec
+create index on gexec_test(a)
+create index on gexec_test(b)
+create index on gexec_test(c)
+create index on gexec_test(d)
+-- \gexec should work in FETCH_COUNT mode too
+-- (though the fetch limit applies to the executed queries not the meta query)
+\set FETCH_COUNT 1
+select 'select 1 as ones', 'select x.y, x.y*2 as double from generate_series(1,4) as x(y)'
+union all
+select 'drop table gexec_test', NULL
+union all
+select 'drop table gexec_test', 'select ''2000-01-01''::date as party_over'
+\gexec
+select 1 as ones
+ ones 
+------
+    1
+(1 row)
+
+select x.y, x.y*2 as double from generate_series(1,4) as x(y)
+ y | double 
+---+--------
+ 1 |      2
+ 2 |      4
+ 3 |      6
+ 4 |      8
+(4 rows)
+
+drop table gexec_test
+drop table gexec_test
+ERROR:  table "gexec_test" does not exist
+select '2000-01-01'::date as party_over
+ party_over 
+------------
+ 01-01-2000
+(1 row)
+
 \unset FETCH_COUNT
 -- show all pset options
 \pset

src/test/regress/sql/psql.sql

@@ -38,6 +38,28 @@ select 10 as test01, 20 as test02 from generate_series(1,0) \gset
 
 \unset FETCH_COUNT
 
+-- \gexec
+
+create temporary table gexec_test(a int, b text, c date, d float);
+select format('create index on gexec_test(%I)', attname)
+from pg_attribute
+where attrelid = 'gexec_test'::regclass and attnum > 0
+order by attnum
+\gexec
+
+-- \gexec should work in FETCH_COUNT mode too
+-- (though the fetch limit applies to the executed queries not the meta query)
+\set FETCH_COUNT 1
+
+select 'select 1 as ones', 'select x.y, x.y*2 as double from generate_series(1,4) as x(y)'
+union all
+select 'drop table gexec_test', NULL
+union all
+select 'drop table gexec_test', 'select ''2000-01-01''::date as party_over'
+\gexec
+
+\unset FETCH_COUNT
+
 -- show all pset options
 \pset