Re-implement pl/pgsql's expression and assignment parsing.

Invent new RawParseModes that allow the core grammar to handle
pl/pgsql expressions and assignments directly, and thereby get rid
of a lot of hackery in pl/pgsql's parser.  This moves a good deal
of knowledge about pl/pgsql into the core code: notably, we have to
invent a CoercionContext that matches pl/pgsql's (rather dubious)
historical behavior for assignment coercions.  That's getting away
from the original idea of pl/pgsql as an arm's-length extension of
the core, but really we crossed that bridge a long time ago.

The main advantage of doing this is that we can now use the core
parser to generate FieldStore and/or SubscriptingRef nodes to handle
assignments to pl/pgsql variables that are records or arrays.  That
fixes a number of cases that had never been implemented in pl/pgsql
assignment, such as nested records and array slicing, and it allows
pl/pgsql assignment to support the datatype-specific subscripting
behaviors introduced in commit c7aba7c14.

There are cosmetic benefits too: when a syntax error occurs in a
pl/pgsql expression, the error report no longer includes the confusing
"SELECT" keyword that used to get prefixed to the expression text.
Also, there seem to be some small speed gains.

Discussion: https://postgr.es/m/4165684.1607707277@sss.pgh.pa.us

2 files changed, 8 insertions, 0 deletions

diff --git a/contrib/hstore/expected/hstore.out b/contrib/hstore/expected/hstore.out
index fdcc3920ce..64a3272b9c 100644
--- a/contrib/hstore/expected/hstore.out
+++ b/contrib/hstore/expected/hstore.out
@@ -1583,6 +1583,10 @@ select f2 from test_json_agg;
  "d"=>NULL, "x"=>"xyzzy"
 (3 rows)
 
+-- Test subscripting in plpgsql
+do $$ declare h hstore;
+begin h['a'] := 'b'; raise notice 'h = %, h[a] = %', h, h['a']; end $$;
+NOTICE:  h = "a"=>"b", h[a] = b
 -- Check the hstore_hash() and hstore_hash_extended() function explicitly.
 SELECT v as value, hstore_hash(v)::bit(32) as standard,
        hstore_hash_extended(v, 0)::bit(32) as extended0,
diff --git a/contrib/hstore/sql/hstore.sql b/contrib/hstore/sql/hstore.sql
index 8d96e30403..a59db66b0a 100644
--- a/contrib/hstore/sql/hstore.sql
+++ b/contrib/hstore/sql/hstore.sql
@@ -372,6 +372,10 @@ select f2['d':'e'] from test_json_agg;  -- error
 update test_json_agg set f2['d'] = f2['e'], f2['x'] = 'xyzzy';
 select f2 from test_json_agg;
 
+-- Test subscripting in plpgsql
+do $$ declare h hstore;
+begin h['a'] := 'b'; raise notice 'h = %, h[a] = %', h, h['a']; end $$;
+
 -- Check the hstore_hash() and hstore_hash_extended() function explicitly.
 SELECT v as value, hstore_hash(v)::bit(32) as standard,
        hstore_hash_extended(v, 0)::bit(32) as extended0,