Improve error message and hint for ALTER COLUMN TYPE can't-cast failure.

We already tried to improve this once, but the "improved" text was rather
off-target if you had provided a USING clause.  Also, it seems helpful
to provide the exact text of a suggested USING clause, so users can just
copy-and-paste it when needed.  Per complaint from Keith Rarick and a
suggestion from Merlin Moncure.

Back-patch to 9.2 where the current wording was adopted.

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 84dbee0c41190e71cdf02473d143e5d88f11a3f9..d3947139c07d58276176ea421ca3e8ac3e8a764c 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -7888,11 +7888,26 @@ ATPrepAlterColumnType(List **wqueue,
                                                                                  COERCE_IMPLICIT_CAST,
                                                                                  -1);
                if (transform == NULL)
-                       ereport(ERROR,
-                                       (errcode(ERRCODE_DATATYPE_MISMATCH),
-                         errmsg("column \"%s\" cannot be cast automatically to type %s",
-                                        colName, format_type_be(targettype)),
-                                        errhint("Specify a USING expression to perform the conversion.")));
+               {
+                       /* error text depends on whether USING was specified or not */
+                       if (def->cooked_default != NULL)
+                               ereport(ERROR,
+                                               (errcode(ERRCODE_DATATYPE_MISMATCH),
+                                                errmsg("result of USING clause for column \"%s\""
+                                                               " cannot be cast automatically to type %s",
+                                                               colName, format_type_be(targettype)),
+                                                errhint("You might need to add an explicit cast.")));
+                       else
+                               ereport(ERROR,
+                                               (errcode(ERRCODE_DATATYPE_MISMATCH),
+                                                errmsg("column \"%s\" cannot be cast automatically to type %s",
+                                                               colName, format_type_be(targettype)),
+                               /* translator: USING is SQL, don't translate it */
+                                          errhint("You might need to specify \"USING %s::%s\".",
+                                                          quote_identifier(colName),
+                                                          format_type_with_typemod(targettype,
+                                                                                                               targettypmod))));
+               }
 
                /* Fix collations after all else */
                assign_expr_collations(pstate, transform);

diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 65274bc26b71cf96a26f92ed08caf294ac1da13b..3ad2c55775c64e61e1f01f8929a04dfb95f5f1c3 100644 (file)
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -1665,7 +1665,7 @@ select f3,max(f1) from foo group by f3;
 -- Simple tests for alter table column type
 alter table foo alter f1 TYPE integer; -- fails
 ERROR:  column "f1" cannot be cast automatically to type integer
-HINT:  Specify a USING expression to perform the conversion.
+HINT:  You might need to specify "USING f1::integer".
 alter table foo alter f1 TYPE varchar(10);
 create table anothertab (atcol1 serial8, atcol2 boolean,
        constraint anothertab_chk check (atcol1 <= 3));
@@ -1680,7 +1680,10 @@ select * from anothertab;
 
 alter table anothertab alter column atcol1 type boolean; -- fails
 ERROR:  column "atcol1" cannot be cast automatically to type boolean
-HINT:  Specify a USING expression to perform the conversion.
+HINT:  You might need to specify "USING atcol1::boolean".
+alter table anothertab alter column atcol1 type boolean using atcol1::int; -- fails
+ERROR:  result of USING clause for column "atcol1" cannot be cast automatically to type boolean
+HINT:  You might need to add an explicit cast.
 alter table anothertab alter column atcol1 type integer;
 select * from anothertab;
  atcol1 | atcol2 

diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index b5ee7b087df8cc225f19c0a866558387ae156d9c..29c1875d2eaec34c58be594aa18fc65330e25828 100644 (file)
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1175,6 +1175,7 @@ insert into anothertab (atcol1, atcol2) values (default, false);
 select * from anothertab;
 
 alter table anothertab alter column atcol1 type boolean; -- fails
+alter table anothertab alter column atcol1 type boolean using atcol1::int; -- fails
 alter table anothertab alter column atcol1 type integer;
 
 select * from anothertab;