v20240404-0011-Implementation-of-DEFAULT-clause-default-expressions.patch

From 77cbd50d678e3f92f7eea359ad92d2a0f1f3c89c Mon Sep 17 00:00:00 2001
From: "okbob@github.com" <okbob@github.com>
Date: Thu, 6 Jul 2023 10:44:20 +0200
Subject: [PATCH 11/19] Implementation of DEFAULT clause - default expressions
 for session variables

When the evaluation of default expression fails, we remove related entry from sessionvars
hash table. Then sessionvars can contain only sucessfully initialized values. Then we don't
need special flag for badly initialized session variables.
---
 doc/src/sgml/catalogs.sgml                    |  8 ++
 doc/src/sgml/ddl.sgml                         | 13 ++-
 doc/src/sgml/ref/create_variable.sgml         | 21 +++--
 doc/src/sgml/ref/discard.sgml                 |  3 +-
 src/backend/catalog/pg_variable.c             | 27 ++++++
 src/backend/commands/session_variable.c       | 87 ++++++++++++++++++-
 src/backend/parser/gram.y                     | 15 +++-
 src/backend/parser/parse_agg.c                |  2 +
 src/backend/parser/parse_expr.c               |  4 +
 src/backend/parser/parse_func.c               |  1 +
 src/bin/pg_dump/pg_dump.c                     | 14 +++
 src/bin/pg_dump/pg_dump.h                     |  1 +
 src/bin/pg_dump/t/002_pg_dump.pl              | 36 ++++++++
 src/bin/psql/describe.c                       |  4 +-
 src/include/catalog/pg_variable.h             |  3 +
 src/include/nodes/parsenodes.h                |  1 +
 src/include/parser/parse_node.h               |  1 +
 src/test/regress/expected/psql.out            | 36 ++++----
 .../regress/expected/session_variables.out    | 73 ++++++++++++++--
 src/test/regress/sql/session_variables.sql    | 48 ++++++++++
 20 files changed, 353 insertions(+), 45 deletions(-)

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 515ed52118..85ba0f69ed 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -9864,6 +9864,14 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>vardefexpr</structfield> <type>pg_node_tree</type>
+      </para>
+      <para>
+       The internal representation of the variable default value
+      </para></entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 62cf852df8..3ae71d6b6b 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -5329,13 +5329,12 @@ SELECT current_user_id;
 
    <para>
     The value of a session variable is local to the current session. Retrieving
-    a variable's value returns a <literal>NULL</literal>, unless its value has
-    been set to something else in the current session using the
-    <command>LET</command> command. The content of a variable is not
-    transactional. This is the same as regular variables in PL languages.
-    The session variables can be persistent or can be temporary. In both cases,
-    the content of session variables is temporary and not shared (like the
-    content of temporary tables).
+    a variable's value returns either a <literal>NULL</literal> or a default
+    value, unless its value has been set to something else in the current
+    session using the <command>LET</command> command. The content of a variable
+    is not transactional. This is the same as regular variables in PL languages.
+    The session variables are persistent, but the content of session variables
+    is temporary and not shared (like the content of temporary tables).
    </para>
 
    <para>
diff --git a/doc/src/sgml/ref/create_variable.sgml b/doc/src/sgml/ref/create_variable.sgml
index 50cf316cdf..6d6e29ac4b 100644
--- a/doc/src/sgml/ref/create_variable.sgml
+++ b/doc/src/sgml/ref/create_variable.sgml
@@ -27,7 +27,7 @@ PostgreSQL documentation
  <refsynopsisdiv>
 <synopsis>
 CREATE [ { TEMPORARY | TEMP } ] VARIABLE [ IF NOT EXISTS ] <replaceable class="parameter">name</replaceable> [ AS ] <replaceable class="parameter">data_type</replaceable> ] [ COLLATE <replaceable class="parameter">collation</replaceable> ]
-    [ { ON COMMIT DROP | ON TRANSACTION END RESET } ]
+    [ DEFAULT <replaceable class="parameter">default_expr</replaceable> ] [ { ON COMMIT DROP | ON TRANSACTION END RESET } ]
 </synopsis>
  </refsynopsisdiv>
  <refsect1>
@@ -42,10 +42,10 @@ CREATE [ { TEMPORARY | TEMP } ] VARIABLE [ IF NOT EXISTS ] <replaceable class="p
 
   <para>
    The value of a session variable is local to the current session. Retrieving
-   a session variable's value returns either a NULL, unless its value is set to
-   something else in the current session with a LET command. The content of
-   a session variable is not transactional. This is the  same as regular
-   variables in PL languages.
+   a session variable's value returns either a NULL or a default value, unless
+   its value is set to something else in the current session with a LET
+   command. The content of a session variable is not transactional. This is the
+   same as regular variables in PL languages.
   </para>
 
   <para>
@@ -111,6 +111,17 @@ CREATE [ { TEMPORARY | TEMP } ] VARIABLE [ IF NOT EXISTS ] <replaceable class="p
     </listitem>
    </varlistentry>
 
+   <varlistentry id="sql-createvariable-default">
+    <term><literal>DEFAULT <replaceable>default_expr</replaceable></literal></term>
+    <listitem>
+     <para>
+      The <literal>DEFAULT</literal> clause can be used to assign a default
+      value to a session variable. This expression is evaluated when the session
+      variable is first accessed for reading and had not yet been assigned a value.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry id="sql-createvariable-on-commit-drop">
     <term><literal>ON COMMIT DROP</literal></term>
     <listitem>
diff --git a/doc/src/sgml/ref/discard.sgml b/doc/src/sgml/ref/discard.sgml
index 61b967f9c9..6b0cb95034 100644
--- a/doc/src/sgml/ref/discard.sgml
+++ b/doc/src/sgml/ref/discard.sgml
@@ -71,7 +71,8 @@ DISCARD { ALL | PLANS | SEQUENCES | TEMPORARY | TEMP | VARIABLES }
     <listitem>
      <para>
       Resets the value of all session variables. If a variable
-      is later reused, it is re-initialized to <literal>NULL</literal>.
+      is later reused, it is re-initialized to either
+      <literal>NULL</literal> or its default value.
      </para>
     </listitem>
    </varlistentry>
diff --git a/src/backend/catalog/pg_variable.c b/src/backend/catalog/pg_variable.c
index e9a6f755fd..6a46461f03 100644
--- a/src/backend/catalog/pg_variable.c
+++ b/src/backend/catalog/pg_variable.c
@@ -24,6 +24,9 @@
 #include "catalog/pg_variable.h"
 #include "commands/session_variable.h"
 #include "miscadmin.h"
+#include "parser/parse_coerce.h"
+#include "parser/parse_collate.h"
+#include "parser/parse_expr.h"
 #include "parser/parse_type.h"
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
@@ -37,6 +40,7 @@ static ObjectAddress create_variable(const char *varName,
 									 Oid varOwner,
 									 Oid varCollation,
 									 bool if_not_exists,
+									 Node *varDefexpr,
 									 VariableEOXAction eoxaction);
 
 
@@ -51,6 +55,7 @@ create_variable(const char *varName,
 				Oid varOwner,
 				Oid varCollation,
 				bool if_not_exists,
+				Node *varDefexpr,
 				VariableEOXAction eoxaction)
 {
 	Acl		   *varacl;
@@ -114,6 +119,11 @@ create_variable(const char *varName,
 	values[Anum_pg_variable_varcollation - 1] = ObjectIdGetDatum(varCollation);
 	values[Anum_pg_variable_vareoxaction - 1] = CharGetDatum(eoxaction);
 
+	if (varDefexpr)
+		values[Anum_pg_variable_vardefexpr - 1] = CStringGetTextDatum(nodeToString(varDefexpr));
+	else
+		nulls[Anum_pg_variable_vardefexpr - 1] = true;
+
 	varacl = get_user_default_acl(OBJECT_VARIABLE, varOwner,
 								  varNamespace);
 	if (varacl != NULL)
@@ -150,6 +160,11 @@ create_variable(const char *varName,
 	record_object_address_dependencies(&myself, addrs, DEPENDENCY_NORMAL);
 	free_object_addresses(addrs);
 
+	/* dependency on default expr */
+	if (varDefexpr)
+		recordDependencyOnExpr(&myself, (Node *) varDefexpr,
+							   NIL, DEPENDENCY_NORMAL);
+
 	/* dependency on owner */
 	recordDependencyOnOwner(VariableRelationId, varid, varOwner);
 
@@ -185,6 +200,7 @@ CreateVariable(ParseState *pstate, CreateSessionVarStmt *stmt)
 	Oid			collation;
 	Oid			typcollation;
 	ObjectAddress variable;
+	Node	   *cooked_default = NULL;
 
 	/* Check consistency of arguments */
 	if (stmt->eoxaction == VARIABLE_EOX_DROP
@@ -226,6 +242,16 @@ CreateVariable(ParseState *pstate, CreateSessionVarStmt *stmt)
 						format_type_be(typid)),
 				 parser_errposition(pstate, stmt->collClause->location)));
 
+	if (stmt->defexpr)
+	{
+		cooked_default = transformExpr(pstate, stmt->defexpr,
+									   EXPR_KIND_VARIABLE_DEFAULT);
+
+		cooked_default = coerce_to_specific_type(pstate,
+												 cooked_default, typid, "DEFAULT");
+		assign_expr_collations(pstate, cooked_default);
+	}
+
 	variable = create_variable(stmt->variable->relname,
 							   namespaceid,
 							   typid,
@@ -233,6 +259,7 @@ CreateVariable(ParseState *pstate, CreateSessionVarStmt *stmt)
 							   varowner,
 							   collation,
 							   stmt->if_not_exists,
+							   cooked_default,
 							   stmt->eoxaction);
 
 	elog(DEBUG1, "record for session variable \"%s\" (oid:%d) was created in pg_variable",
diff --git a/src/backend/commands/session_variable.c b/src/backend/commands/session_variable.c
index fcc9f1616f..a8f63e78a6 100644
--- a/src/backend/commands/session_variable.c
+++ b/src/backend/commands/session_variable.c
@@ -22,6 +22,7 @@
 #include "executor/svariableReceiver.h"
 #include "funcapi.h"
 #include "miscadmin.h"
+#include "optimizer/optimizer.h"
 #include "rewrite/rewriteHandler.h"
 #include "storage/lmgr.h"
 #include "storage/proc.h"
@@ -523,11 +524,68 @@ AtEOSubXact_SessionVariables(bool isCommit,
 	}
 }
 
+/*
+ * evaluate an expression
+ */
+static void
+eval_assign_defexpr(SVariable svar, HeapTuple tup)
+{
+	Datum		defexpr_value;
+	bool		isnull;
+
+	Assert(svar);
+	Assert(svar->is_valid);
+	Assert(HeapTupleIsValid(tup));
+
+	defexpr_value = SysCacheGetAttr(VARIABLEOID,
+									tup,
+									Anum_pg_variable_vardefexpr,
+									&isnull);
+
+	if (!isnull)
+	{
+		EState	   *estate;
+		ExprState  *defexprs;
+		Expr	   *defexpr;
+		char	   *defexpr_str;
+		Datum		value;
+		MemoryContext oldcxt;
+
+		estate = CreateExecutorState();
+
+		defexpr_str = TextDatumGetCString(defexpr_value);
+		defexpr = (Expr *) stringToNode(defexpr_str);
+
+		oldcxt = MemoryContextSwitchTo(estate->es_query_cxt);
+
+		defexpr = expression_planner((Expr *) defexpr);
+		defexprs = ExecInitExpr(defexpr, NULL);
+
+		value = ExecEvalExprSwitchContext(defexprs,
+										  GetPerTupleExprContext(estate),
+										  &isnull);
+
+		MemoryContextSwitchTo(oldcxt);
+
+		if (!isnull)
+		{
+			oldcxt = MemoryContextSwitchTo(SVariableMemoryContext);
+
+			svar->value = datumCopy(value, svar->typbyval, svar->typlen);
+			svar->isnull = false;
+
+			MemoryContextSwitchTo(oldcxt);
+		}
+
+		FreeExecutorState(estate);
+	}
+}
+
 /*
  * Update attributes cached in svar
  */
 static void
-setup_session_variable(SVariable svar, Oid varid)
+setup_session_variable(SVariable svar, Oid varid, bool is_write)
 {
 	HeapTuple	tup;
 	Form_pg_variable varform;
@@ -577,6 +635,9 @@ setup_session_variable(SVariable svar, Oid varid)
 	svar->hashvalue = GetSysCacheHashValue1(VARIABLEOID,
 											ObjectIdGetDatum(varid));
 
+	if (!is_write)
+		eval_assign_defexpr(svar, tup);
+
 	ReleaseSysCache(tup);
 }
 
@@ -606,7 +667,7 @@ set_session_variable(SVariable svar, Datum value, bool isnull)
 	 */
 	if (!svar->is_valid)
 	{
-		setup_session_variable(&locsvar, svar->varid);
+		setup_session_variable(&locsvar, svar->varid, false);
 		_svar = &locsvar;
 	}
 	else
@@ -748,7 +809,25 @@ get_session_variable(Oid varid)
 	 */
 	if (!svar->is_valid)
 	{
-		setup_session_variable(svar, varid);
+		/* in this case we want to use defexp if it is defined */
+		PG_TRY();
+		{
+			/*
+			 * In this case, the setup can execute default expression. When
+			 * the execution of default expression fails, then we need to
+			 * remove entry from session vars.
+			 */
+			setup_session_variable(svar, varid, false);
+		}
+		PG_CATCH();
+		{
+			/* This entry cannot be valid, remove from sessionvars */
+			hash_search(sessionvars, &varid, HASH_REMOVE, NULL);
+
+			/* propagate the error */
+			PG_RE_THROW();
+		}
+		PG_END_TRY();
 
 		elog(DEBUG1, "session variable \"%s.%s\" (oid:%u) has assigned entry in memory (emitted by READ)",
 			 get_namespace_name(get_session_variable_namespace(varid)),
@@ -812,7 +891,7 @@ SetSessionVariable(Oid varid, Datum value, bool isNull)
 
 	if (!found)
 	{
-		setup_session_variable(svar, varid);
+		setup_session_variable(svar, varid, true);
 
 		elog(DEBUG1, "session variable \"%s.%s\" (oid:%u) has assigned entry in memory (emitted by WRITE)",
 			 get_namespace_name(get_session_variable_namespace(svar->varid)),
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 1146b4b695..25eb665d3a 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -652,6 +652,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <partboundspec> PartitionBoundSpec
 %type <list>		hash_partbound
 %type <defelt>		hash_partbound_elem
+%type <node>		OptSessionVarDefExpr
 
 %type <node>	json_format_clause
 				json_format_clause_opt
@@ -5154,30 +5155,36 @@ create_extension_opt_item:
  *****************************************************************************/
 
 CreateSessionVarStmt:
-			CREATE OptTemp VARIABLE qualified_name opt_as Typename opt_collate_clause OnEOXActionOption
+			CREATE OptTemp VARIABLE qualified_name opt_as Typename opt_collate_clause OptSessionVarDefExpr OnEOXActionOption
 				{
 					CreateSessionVarStmt *n = makeNode(CreateSessionVarStmt);
 					$4->relpersistence = $2;
 					n->variable = $4;
 					n->typeName = $6;
 					n->collClause = (CollateClause *) $7;
-					n->eoxaction = $8;
+					n->defexpr = $8;
+					n->eoxaction = $9;
 					n->if_not_exists = false;
 					$$ = (Node *) n;
 				}
-			| CREATE OptTemp VARIABLE IF_P NOT EXISTS qualified_name opt_as Typename opt_collate_clause OnEOXActionOption
+			| CREATE OptTemp VARIABLE IF_P NOT EXISTS qualified_name opt_as Typename opt_collate_clause OptSessionVarDefExpr OnEOXActionOption
 				{
 					CreateSessionVarStmt *n = makeNode(CreateSessionVarStmt);
 					$7->relpersistence = $2;
 					n->variable = $7;
 					n->typeName = $9;
 					n->collClause = (CollateClause *) $10;
-					n->eoxaction = $11;
+					n->defexpr = $11;
+					n->eoxaction = $12;
 					n->if_not_exists = true;
 					$$ = (Node *) n;
 				}
 		;
 
+OptSessionVarDefExpr: DEFAULT b_expr					{ $$ = $2; }
+			| /* EMPTY */							{ $$ = NULL; }
+		;
+
 /*
  * Temporary session variables can be dropped on successful
  * transaction end like tables.
diff --git a/src/backend/parser/parse_agg.c b/src/backend/parser/parse_agg.c
index b494c885e9..cb57203e18 100644
--- a/src/backend/parser/parse_agg.c
+++ b/src/backend/parser/parse_agg.c
@@ -485,6 +485,7 @@ check_agglevels_and_constraints(ParseState *pstate, Node *expr)
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 
 			if (isAgg)
 				err = _("aggregate functions are not allowed in DEFAULT expressions");
@@ -934,6 +935,7 @@ transformWindowFuncCall(ParseState *pstate, WindowFunc *wfunc,
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			err = _("window functions are not allowed in DEFAULT expressions");
 			break;
 		case EXPR_KIND_INDEX_EXPRESSION:
diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index 54fa89334c..c70517c3c7 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -583,6 +583,7 @@ expr_kind_allows_session_variables(ParseExprKind p_expr_kind)
 		case EXPR_KIND_JOIN_USING:
 		case EXPR_KIND_CYCLE_MARK:
 		case EXPR_KIND_ASSIGN_TARGET:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			result = false;
 			break;
 	}
@@ -668,6 +669,7 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 		case EXPR_KIND_CYCLE_MARK:
 		case EXPR_KIND_ASSIGN_TARGET:
 		case EXPR_KIND_LET_TARGET:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			/* okay */
 			break;
 
@@ -2040,6 +2042,7 @@ transformSubLink(ParseState *pstate, SubLink *sublink)
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			err = _("cannot use subquery in DEFAULT expression");
 			break;
 		case EXPR_KIND_INDEX_EXPRESSION:
@@ -3390,6 +3393,7 @@ ParseExprKindName(ParseExprKind exprKind)
 			return "CHECK";
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			return "DEFAULT";
 		case EXPR_KIND_INDEX_EXPRESSION:
 			return "index expression";
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 2dc70ef990..b53263e662 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2619,6 +2619,7 @@ check_srf_call_placement(ParseState *pstate, Node *last_srf, int location)
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			err = _("set-returning functions are not allowed in DEFAULT expressions");
 			break;
 		case EXPR_KIND_INDEX_EXPRESSION:
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index 6f0b0eb4cd..cc81ac7fd1 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -5312,6 +5312,7 @@ getVariables(Archive *fout)
 	int			i_varnamespace;
 	int			i_vartype;
 	int			i_vartypname;
+	int			i_vardefexpr;
 	int			i_vareoxaction;
 	int			i_varowner;
 	int			i_varcollation;
@@ -5336,6 +5337,7 @@ getVariables(Archive *fout)
 					  "pg_catalog.format_type(v.vartype, v.vartypmod) as vartypname,\n"
 					  "CASE WHEN v.varcollation <> t.typcollation "
 					  "THEN v.varcollation ELSE 0 END AS varcollation,\n"
+					  "pg_catalog.pg_get_expr(v.vardefexpr,0) as vardefexpr,\n"
 					  "v.varowner,\n"
 					  "v.varacl,\n"
 					  "acldefault('V', v.varowner) AS acldefault\n"
@@ -5353,6 +5355,7 @@ getVariables(Archive *fout)
 	i_varnamespace = PQfnumber(res, "varnamespace");
 	i_vartype = PQfnumber(res, "vartype");
 	i_vartypname = PQfnumber(res, "vartypname");
+	i_vardefexpr = PQfnumber(res, "vardefexpr");
 	i_vareoxaction = PQfnumber(res, "vareoxaction");
 	i_varcollation = PQfnumber(res, "varcollation");
 
@@ -5389,6 +5392,11 @@ getVariables(Archive *fout)
 		/* Decide whether we want to dump it */
 		selectDumpableObject(&(varinfo[i].dobj), fout);
 
+		if (PQgetisnull(res, i, i_vardefexpr))
+			varinfo[i].vardefexpr = NULL;
+		else
+			varinfo[i].vardefexpr = pg_strdup(PQgetvalue(res, i, i_vardefexpr));
+
 		/* Do not try to dump ACL if no ACL exists. */
 		if (!PQgetisnull(res, i, i_varacl))
 			varinfo[i].dobj.components |= DUMP_COMPONENT_ACL;
@@ -5421,6 +5429,7 @@ dumpVariable(Archive *fout, const VariableInfo *varinfo)
 	PQExpBuffer query;
 	char	   *qualvarname;
 	const char *vartypname;
+	const char *vardefexpr;
 	const char *vareoxaction;
 	Oid			varcollation;
 
@@ -5433,6 +5442,7 @@ dumpVariable(Archive *fout, const VariableInfo *varinfo)
 
 	qualvarname = pg_strdup(fmtQualifiedDumpable(varinfo));
 	vartypname = varinfo->vartypname;
+	vardefexpr = varinfo->vardefexpr;
 	vareoxaction = varinfo->vareoxaction;
 	varcollation = varinfo->varcollation;
 
@@ -5452,6 +5462,10 @@ dumpVariable(Archive *fout, const VariableInfo *varinfo)
 							  fmtQualifiedDumpable(coll));
 	}
 
+	if (vardefexpr)
+		appendPQExpBuffer(query, " DEFAULT %s",
+						  vardefexpr);
+
 	if (strcmp(vareoxaction, "r") == 0)
 		appendPQExpBuffer(query, " ON TRANSACTION END RESET");
 
diff --git a/src/bin/pg_dump/pg_dump.h b/src/bin/pg_dump/pg_dump.h
index 2556cdac57..3ad836bff1 100644
--- a/src/bin/pg_dump/pg_dump.h
+++ b/src/bin/pg_dump/pg_dump.h
@@ -708,6 +708,7 @@ typedef struct _VariableInfo
 	DumpableAcl dacl;
 	Oid			vartype;
 	char	   *vartypname;
+	char	   *vardefexpr;
 	char	   *vareoxaction;
 	char	   *varacl;
 	char	   *rvaracl;
diff --git a/src/bin/pg_dump/t/002_pg_dump.pl b/src/bin/pg_dump/t/002_pg_dump.pl
index 4cf49d4875..96dffc1a51 100644
--- a/src/bin/pg_dump/t/002_pg_dump.pl
+++ b/src/bin/pg_dump/t/002_pg_dump.pl
@@ -3997,6 +3997,42 @@ my %tests = (
 		},
 	},
 
+	'CREATE VARIABLE test_variable DEFAULT' => {
+		all_runs     => 1,
+		catch_all    => 'CREATE ... commands',
+		create_order => 61,
+		create_sql   => 'CREATE VARIABLE dump_test.variable3 AS integer DEFAULT 10;',
+		regexp => qr/^
+			\QCREATE VARIABLE dump_test.variable3 AS integer DEFAULT 10;\E/xm,
+		like => {
+			%full_runs,
+			%dump_test_schema_runs,
+			section_pre_data => 1,
+		},
+		unlike => {
+			exclude_dump_test_schema => 1,
+			only_dump_measurement    => 1,
+		},
+	},
+
+	'CREATE VARIABLE test_variable DEFAULT ON TRANSACTION END RESET' => {
+		all_runs     => 1,
+		catch_all    => 'CREATE ... commands',
+		create_order => 61,
+		create_sql   => 'CREATE VARIABLE dump_test.variable4 AS integer DEFAULT 10 ON TRANSACTION END RESET',
+		regexp => qr/^
+			\QCREATE VARIABLE dump_test.variable4 AS integer DEFAULT 10 ON TRANSACTION END RESET;\E/xm,
+		like => {
+			%full_runs,
+			%dump_test_schema_runs,
+			section_pre_data => 1,
+		},
+		unlike => {
+			exclude_dump_test_schema => 1,
+			only_dump_measurement    => 1,
+		},
+	},
+
 	'CREATE VIEW test_view' => {
 		create_order => 61,
 		create_sql => 'CREATE VIEW dump_test.test_view
diff --git a/src/bin/psql/describe.c b/src/bin/psql/describe.c
index 5d492aa05d..819a2b7e0f 100644
--- a/src/bin/psql/describe.c
+++ b/src/bin/psql/describe.c
@@ -5201,7 +5201,7 @@ listVariables(const char *pattern, bool verbose)
 	PQExpBufferData buf;
 	PGresult   *res;
 	printQueryOpt myopt = pset.popt;
-	static const bool translate_columns[] = {false, false, false, false, false, false, false, false};
+	static const bool translate_columns[] = {false, false, false, false, false, false, false, false, false};
 
 	initPQExpBuffer(&buf);
 
@@ -5212,6 +5212,7 @@ listVariables(const char *pattern, bool verbose)
 					  "  (SELECT c.collname FROM pg_catalog.pg_collation c, pg_catalog.pg_type bt\n"
 					  "   WHERE c.oid = v.varcollation AND bt.oid = v.vartype AND v.varcollation <> bt.typcollation) as \"%s\",\n"
 					  "  pg_catalog.pg_get_userbyid(v.varowner) as \"%s\",\n"
+					  "  pg_catalog.pg_get_expr(v.vardefexpr, 0) as \"%s\",\n"
 					  "  CASE v.vareoxaction\n"
 					  "    WHEN 'd' THEN 'ON COMMIT DROP'\n"
 					  "    WHEN 'r' THEN 'ON TRANSACTION END RESET'\n"
@@ -5221,6 +5222,7 @@ listVariables(const char *pattern, bool verbose)
 					  gettext_noop("Type"),
 					  gettext_noop("Collation"),
 					  gettext_noop("Owner"),
+					  gettext_noop("Default"),
 					  gettext_noop("Transactional end action"));
 
 	if (verbose)
diff --git a/src/include/catalog/pg_variable.h b/src/include/catalog/pg_variable.h
index 2aecbcb50e..3d8cbff50d 100644
--- a/src/include/catalog/pg_variable.h
+++ b/src/include/catalog/pg_variable.h
@@ -66,6 +66,9 @@ CATALOG(pg_variable,9222,VariableRelationId)
 	/* access permissions */
 	aclitem		varacl[1] BKI_DEFAULT(_null_);
 
+	/* list of expression trees for variable default (NULL if none) */
+	pg_node_tree vardefexpr BKI_DEFAULT(_null_);
+
 #endif
 } FormData_pg_variable;
 
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 3fa0ae107c..30141d76f4 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -3377,6 +3377,7 @@ typedef struct CreateSessionVarStmt
 	TypeName   *typeName;		/* the type of variable */
 	CollateClause *collClause;
 	bool		if_not_exists;	/* do nothing if it already exists */
+	Node	   *defexpr;		/* default expression */
 	char		eoxaction;		/* on commit action */
 } CreateSessionVarStmt;
 
diff --git a/src/include/parser/parse_node.h b/src/include/parser/parse_node.h
index 6d93691965..baba22ed84 100644
--- a/src/include/parser/parse_node.h
+++ b/src/include/parser/parse_node.h
@@ -84,6 +84,7 @@ typedef enum ParseExprKind
 	EXPR_KIND_CYCLE_MARK,		/* cycle mark value */
 	EXPR_KIND_ASSIGN_TARGET,	/* PL/pgSQL assignment target */
 	EXPR_KIND_LET_TARGET,		/* LET target */
+	EXPR_KIND_VARIABLE_DEFAULT, /* default value for session variable */
 } ParseExprKind;
 
 
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 112702a0bc..ddca38d915 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -5830,20 +5830,20 @@ CREATE ROLE regress_variable_owner;
 SET ROLE TO regress_variable_owner;
 CREATE VARIABLE var1 AS varchar COLLATE "C";
 \dV+ var1
-                                                          List of variables
- Schema | Name |       Type        | Collation |         Owner          | Transactional end action | Access privileges | Description 
---------+------+-------------------+-----------+------------------------+--------------------------+-------------------+-------------
- public | var1 | character varying | C         | regress_variable_owner |                          |                   | 
+                                                               List of variables
+ Schema | Name |       Type        | Collation |         Owner          | Default | Transactional end action | Access privileges | Description 
+--------+------+-------------------+-----------+------------------------+---------+--------------------------+-------------------+-------------
+ public | var1 | character varying | C         | regress_variable_owner |         |                          |                   | 
 (1 row)
 
 GRANT SELECT ON VARIABLE var1 TO PUBLIC;
 COMMENT ON VARIABLE var1 IS 'some description';
 \dV+ var1
-                                                                            List of variables
- Schema | Name |       Type        | Collation |         Owner          | Transactional end action |                Access privileges                 |   Description    
---------+------+-------------------+-----------+------------------------+--------------------------+--------------------------------------------------+------------------
- public | var1 | character varying | C         | regress_variable_owner |                          | regress_variable_owner=rw/regress_variable_owner+| some description
-        |      |                   |           |                        |                          | =r/regress_variable_owner                        | 
+                                                                                 List of variables
+ Schema | Name |       Type        | Collation |         Owner          | Default | Transactional end action |                Access privileges                 |   Description    
+--------+------+-------------------+-----------+------------------------+---------+--------------------------+--------------------------------------------------+------------------
+ public | var1 | character varying | C         | regress_variable_owner |         |                          | regress_variable_owner=rw/regress_variable_owner+| some description
+        |      |                   |           |                        |         |                          | =r/regress_variable_owner                        | 
 (1 row)
 
 DROP VARIABLE var1;
@@ -6311,9 +6311,9 @@ List of schemas
 (0 rows)
 
 \dV "no.such.variable"
-                          List of variables
- Schema | Name | Type | Collation | Owner | Transactional end action 
---------+------+------+-----------+-------+--------------------------
+                               List of variables
+ Schema | Name | Type | Collation | Owner | Default | Transactional end action 
+--------+------+------+-----------+-------+---------+--------------------------
 (0 rows)
 
 -- again, but with dotted schema qualifications.
@@ -6486,9 +6486,9 @@ improper qualified name (too many dotted names): "no.such.schema"."no.such.insta
 \dy "no.such.schema"."no.such.event.trigger"
 improper qualified name (too many dotted names): "no.such.schema"."no.such.event.trigger"
 \dV "no.such.schema"."no.such.variable"
-                          List of variables
- Schema | Name | Type | Collation | Owner | Transactional end action 
---------+------+------+-----------+-------+--------------------------
+                               List of variables
+ Schema | Name | Type | Collation | Owner | Default | Transactional end action 
+--------+------+------+-----------+-------+---------+--------------------------
 (0 rows)
 
 -- again, but with current database and dotted schema qualifications.
@@ -6625,9 +6625,9 @@ List of text search templates
 (0 rows)
 
 \dV regression."no.such.schema"."no.such.variable"
-                          List of variables
- Schema | Name | Type | Collation | Owner | Transactional end action 
---------+------+------+-----------+-------+--------------------------
+                               List of variables
+ Schema | Name | Type | Collation | Owner | Default | Transactional end action 
+--------+------+------+-----------+-------+---------+--------------------------
 (0 rows)
 
 -- again, but with dotted database and dotted schema qualifications.
diff --git a/src/test/regress/expected/session_variables.out b/src/test/regress/expected/session_variables.out
index ae912554ad..7e7c1b745a 100644
--- a/src/test/regress/expected/session_variables.out
+++ b/src/test/regress/expected/session_variables.out
@@ -45,11 +45,11 @@ SET ROLE TO regress_variable_owner;
 CREATE VARIABLE svartest.var1 AS int;
 SET ROLE TO DEFAULT;
 \dV+ svartest.var1
-                                                                     List of variables
-  Schema  | Name |  Type   | Collation |         Owner          | Transactional end action |                Access privileges                 | Description 
-----------+------+---------+-----------+------------------------+--------------------------+--------------------------------------------------+-------------
- svartest | var1 | integer |           | regress_variable_owner |                          | regress_variable_owner=rw/regress_variable_owner+| 
-          |      |         |           |                        |                          | regress_variable_reader=r/regress_variable_owner | 
+                                                                          List of variables
+  Schema  | Name |  Type   | Collation |         Owner          | Default | Transactional end action |                Access privileges                 | Description 
+----------+------+---------+-----------+------------------------+---------+--------------------------+--------------------------------------------------+-------------
+ svartest | var1 | integer |           | regress_variable_owner |         |                          | regress_variable_owner=rw/regress_variable_owner+| 
+          |      |         |           |                        |         |                          | regress_variable_reader=r/regress_variable_owner | 
 (1 row)
 
 DROP VARIABLE svartest.var1;
@@ -1494,3 +1494,66 @@ SELECT var1 IS NULL;
 (1 row)
 
 DROP VARIABLE var1;
+CREATE OR REPLACE FUNCTION vartest_fx()
+RETURNS int AS $$
+BEGIN
+  RAISE NOTICE 'vartest_fx executed';
+  RETURN 0;
+END;
+$$ LANGUAGE plpgsql;
+CREATE VARIABLE var1 AS int DEFAULT vartest_fx();
+-- vartest_fx should be protected by dep, should fail
+DROP FUNCTION vartest_fx();
+ERROR:  cannot drop function vartest_fx() because other objects depend on it
+DETAIL:  session variable var1 depends on function vartest_fx()
+HINT:  Use DROP ... CASCADE to drop the dependent objects too.
+-- should be ok
+SELECT var1;
+NOTICE:  vartest_fx executed
+ var1 
+------
+    0
+(1 row)
+
+-- the defexpr should be evaluated only once
+SELECT var1;
+ var1 
+------
+    0
+(1 row)
+
+DISCARD VARIABLES;
+-- in this case, the defexpr should not be evaluated
+LET var1 = 100;
+SELECT var1;
+ var1 
+------
+  100
+(1 row)
+
+DISCARD VARIABLES;
+CREATE OR REPLACE FUNCTION vartest_fx()
+RETURNS int AS $$
+BEGIN
+  RAISE EXCEPTION 'vartest_fx is executing';
+  RETURN 0;
+END;
+$$ LANGUAGE plpgsql;
+-- should to fail, but not to crash
+SELECT var1;
+ERROR:  vartest_fx is executing
+CONTEXT:  PL/pgSQL function vartest_fx() line 3 at RAISE
+-- again
+SELECT var1;
+ERROR:  vartest_fx is executing
+CONTEXT:  PL/pgSQL function vartest_fx() line 3 at RAISE
+-- but we can write
+LET var1 = 100;
+SELECT var1;
+ var1 
+------
+  100
+(1 row)
+
+DROP VARIABLE var1;
+DROP FUNCTION vartest_fx();
diff --git a/src/test/regress/sql/session_variables.sql b/src/test/regress/sql/session_variables.sql
index 91b3472b88..6f9e7e1a57 100644
--- a/src/test/regress/sql/session_variables.sql
+++ b/src/test/regress/sql/session_variables.sql
@@ -984,3 +984,51 @@ ROLLBACK;
 SELECT var1 IS NULL;
 
 DROP VARIABLE var1;
+
+CREATE OR REPLACE FUNCTION vartest_fx()
+RETURNS int AS $$
+BEGIN
+  RAISE NOTICE 'vartest_fx executed';
+  RETURN 0;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE VARIABLE var1 AS int DEFAULT vartest_fx();
+
+-- vartest_fx should be protected by dep, should fail
+DROP FUNCTION vartest_fx();
+
+-- should be ok
+SELECT var1;
+
+-- the defexpr should be evaluated only once
+SELECT var1;
+
+DISCARD VARIABLES;
+
+-- in this case, the defexpr should not be evaluated
+LET var1 = 100;
+SELECT var1;
+
+DISCARD VARIABLES;
+
+CREATE OR REPLACE FUNCTION vartest_fx()
+RETURNS int AS $$
+BEGIN
+  RAISE EXCEPTION 'vartest_fx is executing';
+  RETURN 0;
+END;
+$$ LANGUAGE plpgsql;
+
+-- should to fail, but not to crash
+SELECT var1;
+
+-- again
+SELECT var1;
+
+-- but we can write
+LET var1 = 100;
+SELECT var1;
+
+DROP VARIABLE var1;
+DROP FUNCTION vartest_fx();
-- 
2.44.0