From 0382f4b331a9dffff0a2b8479045805b84012027 Mon Sep 17 00:00:00 2001
From: Brent Moran <brentmoran@gmail.com>
Date: Thu, 12 Dec 2024 23:25:26 +0800
Subject: [PATCH] extract default describing function, add execution logic

---
 db/sql/00_msar.sql      | 55 ++++++++++++++++++++++++++++++-----------
 db/sql/test_00_msar.sql |  2 +-
 2 files changed, 42 insertions(+), 15 deletions(-)

diff --git a/db/sql/00_msar.sql b/db/sql/00_msar.sql
index 64563da1ba..b217227b6a 100644
--- a/db/sql/00_msar.sql
+++ b/db/sql/00_msar.sql
@@ -934,6 +934,46 @@ WHERE has_privilege;
 $$ LANGUAGE SQL STABLE RETURNS NULL ON NULL INPUT;
 
 
+CREATE OR REPLACE FUNCTION
+msar.describe_column_default(tab_id regclass, col_id smallint) RETURNS jsonb AS $$/*
+Return a JSONB object describing the default (if any) of the given column in the given table.
+
+The returned JSON will have the form:
+  {
+    "value": <any>,
+    "is_dynamic": <bool>,
+  }
+
+If the default is possibly dynamic, i.e., if "is_dynamic" is true, then "value" will be a text SQL
+expression that generates the default value if evaluated. If it is not dynamic, then "value" is the
+actual default value.
+*/
+DECLARE
+  def_expr text;
+  def_json jsonb;
+BEGIN
+def_expr = CASE
+  WHEN attidentity='' THEN pg_catalog.pg_get_expr(adbin, tab_id)
+  ELSE 'identity'
+END
+FROM pg_catalog.pg_attribute LEFT JOIN pg_catalog.pg_attrdef ON attrelid=adrelid AND attnum=adnum
+WHERE attrelid=tab_id AND attnum=col_id;
+IF def_expr IS NULL THEN
+  RETURN NULL;
+ELSIF msar.is_default_possibly_dynamic(tab_id, col_id) THEN
+  EXECUTE format(
+    'SELECT jsonb_build_object(''value'', %L, ''is_dynamic'', true)', def_expr
+  ) INTO def_json;
+ELSE
+  EXECUTE format(
+    'SELECT jsonb_build_object(''value'', msar.format_data(%s), ''is_dynamic'', false)', def_expr
+  ) INTO def_json;
+END IF;
+RETURN def_json;
+END;
+$$ LANGUAGE plpgsql RETURNS NULL ON NULL INPUT;
+
+
 CREATE OR REPLACE FUNCTION msar.get_column_info(tab_id regclass) RETURNS jsonb AS $$/*
 Given a table identifier, return an array of objects describing the columns of the table.
 
@@ -965,20 +1005,7 @@ SELECT jsonb_agg(
     'type_options', msar.get_type_options(atttypid, atttypmod, attndims),
     'nullable', NOT attnotnull,
     'primary_key', COALESCE(pgi.indisprimary, false),
-    'default',
-    nullif(
-      jsonb_strip_nulls(
-        jsonb_build_object(
-          'value',
-          CASE
-            WHEN attidentity='' THEN pg_get_expr(adbin, tab_id)
-            ELSE 'identity'
-          END,
-          'is_dynamic', msar.is_default_possibly_dynamic(tab_id, attnum)
-        )
-      ),
-      jsonb_build_object()
-    ),
+    'default', msar.describe_column_default(tab_id, attnum),
     'has_dependents', msar.has_dependents(tab_id, attnum),
     'description', msar.col_description(tab_id, attnum),
     'current_role_priv', msar.list_column_privileges_for_current_role(tab_id, attnum),
diff --git a/db/sql/test_00_msar.sql b/db/sql/test_00_msar.sql
index 820e4a4545..924eb9c30c 100644
--- a/db/sql/test_00_msar.sql
+++ b/db/sql/test_00_msar.sql
@@ -2662,7 +2662,7 @@ BEGIN
         "name": "txt",
         "type": "text",
         "default": {
-          "value": "'abc'::text",
+          "value": "abc",
           "is_dynamic": false
         },
         "nullable": true,