Add new OID alias type regdatabase.

This provides a convenient way to look up a database's OID.  For
example, the query

    SELECT * FROM pg_shdepend
    WHERE dbid = (SELECT oid FROM pg_database
                  WHERE datname = current_database());

can now be simplified to

    SELECT * FROM pg_shdepend
    WHERE dbid = current_database()::regdatabase;

Like the regrole type, regdatabase has cluster-wide scope, so we
disallow regdatabase constants from appearing in stored
expressions.

Bumps catversion.

Author: Ian Lawrence Barwick <barwick@gmail.com>
Reviewed-by: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/aBpjJhyHpM2LYcG0%40nathan

src/backend/bootstrap/bootstrap.c

@@ -109,6 +109,8 @@ static const struct typinfo TypInfo[] = {
 	F_REGROLEIN, F_REGROLEOUT},
 	{"regnamespace", REGNAMESPACEOID, 0, 4, true, TYPALIGN_INT, TYPSTORAGE_PLAIN, InvalidOid,
 	F_REGNAMESPACEIN, F_REGNAMESPACEOUT},
+	{"regdatabase", REGDATABASEOID, 0, 4, true, TYPALIGN_INT, TYPSTORAGE_PLAIN, InvalidOid,
+	F_REGDATABASEIN, F_REGDATABASEOUT},
 	{"text", TEXTOID, 0, -1, false, TYPALIGN_INT, TYPSTORAGE_EXTENDED, DEFAULT_COLLATION_OID,
 	F_TEXTIN, F_TEXTOUT},
 	{"oid", OIDOID, 0, 4, true, TYPALIGN_INT, TYPSTORAGE_PLAIN, InvalidOid,

src/backend/catalog/dependency.c

@@ -1850,6 +1850,17 @@ find_expr_references_walker(Node *node,
 							 errmsg("constant of the type %s cannot be used here",
 									"regrole")));
 					break;
+
+					/*
+					 * Dependencies for regdatabase should be shared among all
+					 * databases, so explicitly inhibit to have dependencies.
+					 */
+				case REGDATABASEOID:
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("constant of the type %s cannot be used here",
+									"regdatabase")));
+					break;
 			}
 		}
 		return false;

src/backend/utils/adt/regproc.c

@@ -30,6 +30,7 @@
 #include "catalog/pg_ts_config.h"
 #include "catalog/pg_ts_dict.h"
 #include "catalog/pg_type.h"
+#include "commands/dbcommands.h"
 #include "lib/stringinfo.h"
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
@@ -1763,6 +1764,123 @@ regnamespacesend(PG_FUNCTION_ARGS)
 	return oidsend(fcinfo);
 }
 
+/*
+ * regdatabasein - converts database name to database OID
+ *
+ * We also accept a numeric OID, for symmetry with the output routine.
+ *
+ * '-' signifies unknown (OID 0).  In all other cases, the input must
+ * match an existing pg_database entry.
+ */
+Datum
+regdatabasein(PG_FUNCTION_ARGS)
+{
+	char	   *db_name_or_oid = PG_GETARG_CSTRING(0);
+	Node	   *escontext = fcinfo->context;
+	Oid			result;
+	List	   *names;
+
+	/* Handle "-" or numeric OID */
+	if (parseDashOrOid(db_name_or_oid, &result, escontext))
+		PG_RETURN_OID(result);
+
+	/* The rest of this wouldn't work in bootstrap mode */
+	if (IsBootstrapProcessingMode())
+		elog(ERROR, "regdatabase values must be OIDs in bootstrap mode");
+
+	/* Normal case: see if the name matches any pg_database entry. */
+	names = stringToQualifiedNameList(db_name_or_oid, escontext);
+	if (names == NIL)
+		PG_RETURN_NULL();
+
+	if (list_length(names) != 1)
+		ereturn(escontext, (Datum) 0,
+				(errcode(ERRCODE_INVALID_NAME),
+				 errmsg("invalid name syntax")));
+
+	result = get_database_oid(strVal(linitial(names)), true);
+
+	if (!OidIsValid(result))
+		ereturn(escontext, (Datum) 0,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("database \"%s\" does not exist",
+						strVal(linitial(names)))));
+
+	PG_RETURN_OID(result);
+}
+
+/*
+ * to_regdatabase - converts database name to database OID
+ *
+ * If the name is not found, we return NULL.
+ */
+Datum
+to_regdatabase(PG_FUNCTION_ARGS)
+{
+	char	   *db_name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	Datum		result;
+	ErrorSaveContext escontext = {T_ErrorSaveContext};
+
+	if (!DirectInputFunctionCallSafe(regdatabasein, db_name,
+									 InvalidOid, -1,
+									 (Node *) &escontext,
+									 &result))
+		PG_RETURN_NULL();
+	PG_RETURN_DATUM(result);
+}
+
+/*
+ * regdatabaseout - converts database OID to database name
+ */
+Datum
+regdatabaseout(PG_FUNCTION_ARGS)
+{
+	Oid			dboid = PG_GETARG_OID(0);
+	char	   *result;
+
+	if (dboid == InvalidOid)
+	{
+		result = pstrdup("-");
+		PG_RETURN_CSTRING(result);
+	}
+
+	result = get_database_name(dboid);
+
+	if (result)
+	{
+		/* pstrdup is not really necessary, but it avoids a compiler warning */
+		result = pstrdup(quote_identifier(result));
+	}
+	else
+	{
+		/* If OID doesn't match any database, return it numerically */
+		result = (char *) palloc(NAMEDATALEN);
+		snprintf(result, NAMEDATALEN, "%u", dboid);
+	}
+
+	PG_RETURN_CSTRING(result);
+}
+
+/*
+ * regdatabaserecv - converts external binary format to regdatabase
+ */
+Datum
+regdatabaserecv(PG_FUNCTION_ARGS)
+{
+	/* Exactly the same as oidrecv, so share code */
+	return oidrecv(fcinfo);
+}
+
+/*
+ * regdatabasesend - converts regdatabase to binary format
+ */
+Datum
+regdatabasesend(PG_FUNCTION_ARGS)
+{
+	/* Exactly the same as oidsend, so share code */
+	return oidsend(fcinfo);
+}
+
 /*
  * text_regclass: convert text to regclass
  *

src/backend/utils/adt/selfuncs.c

@@ -4619,6 +4619,7 @@ convert_to_scalar(Datum value, Oid valuetypid, Oid collid, double *scaledvalue,
 		case REGDICTIONARYOID:
 		case REGROLEOID:
 		case REGNAMESPACEOID:
+		case REGDATABASEOID:
 			*scaledvalue = convert_numeric_to_scalar(value, valuetypid,
 													 &failure);
 			*scaledlobound = convert_numeric_to_scalar(lobound, boundstypid,
@@ -4751,6 +4752,7 @@ convert_numeric_to_scalar(Datum value, Oid typid, bool *failure)
 		case REGDICTIONARYOID:
 		case REGROLEOID:
 		case REGNAMESPACEOID:
+		case REGDATABASEOID:
 			/* we can treat OIDs as integers... */
 			return (double) DatumGetObjectId(value);
 	}

src/backend/utils/cache/catcache.c

@@ -317,6 +317,7 @@ GetCCHashEqFuncs(Oid keytype, CCHashFN *hashfunc, RegProcedure *eqfunc, CCFastEq
 		case REGDICTIONARYOID:
 		case REGROLEOID:
 		case REGNAMESPACEOID:
+		case REGDATABASEOID:
 			*hashfunc = int4hashfast;
 			*fasteqfunc = int4eqfast;
 			*eqfunc = F_OIDEQ;