Large OBjectS) are stored within a table, and if that entry is changed, the
associated BLOB is deleted from the database.
-As PostgreSQL stands, this doesn't occur. It allocates an OID for each object,
-and it is up to the application to store, and ultimately delete the objects.
-
-Now this is fine for new postgresql specific applications, but existing ones
-using JDBC or ODBC wont delete the objects, arising to orphaning - objects
+As PostgreSQL stands, this doesn't occur. Large objects are treated as
+objects in their own right; a table entry can reference a large object by
+OID, but there can be multiple table entries referencing the same large
+object OID, so the system doesn't delete the large object just because you
+change or remove one such entry.
+
+Now this is fine for new PostgreSQL-specific applications, but existing ones
+using JDBC or ODBC won't delete the objects, resulting in orphaning - objects
that are not referenced by anything, and simply occupy disk space.
+
The Fix
I've fixed this by creating a new data type 'lo', some support functions, and
-a Trigger which handles the orphaning problem.
+a Trigger which handles the orphaning problem. The trigger essentially just
+does a 'lo_unlink' whenever you delete or modify a value referencing a large
+object. When you use this trigger, you are assuming that there is only one
+database reference to any large object that is referenced in a
+trigger-controlled column!
+
+The 'lo' type was created because we needed to differentiate between plain
+OIDs and Large Objects. Currently the JDBC driver handles this dilemma easily,
+but (after talking to Byron), the ODBC driver needed a unique type. They had
+created an 'lo' type, but not the solution to orphaning.
+
+You don't actually have to use the 'lo' type to use the trigger, but it may be
+convenient to use it to keep track of which columns in your database represent
+large objects that you are managing with the trigger.
-The 'lo' type was created because we needed to differenciate between normal
-Oid's and Large Objects. Currently the JDBC driver handles this dilema easily,
-but (after talking to Byron), the ODBC driver needed a unique type. They had created an 'lo' type, but not the solution to orphaning.
Install
Ok, first build the shared library, and install. Typing 'make install' in the
contrib/lo directory should do it.
-Then, as the postgres super user, run the lo.sql script. This will install the
-type, and define the support functions.
+Then, as the postgres super user, run the lo.sql script in any database that
+needs the features. This will install the type, and define the support
+functions. You can run the script once in template1, and the objects will be
+inherited by subsequently-created databases.
+
How to Use
The easiest way is by an example:
-> create table image (title text,raster lo);
-> create trigger t_image before update or delete on image for each row execute procedure lo_manage(raster);
+> create table image (title text, raster lo);
+> create trigger t_raster before update or delete on image
+> for each row execute procedure lo_manage(raster);
-Here, a trigger is created for each column that contains a lo type.
+Create a trigger for each column that contains a lo type, and give the column
+name as the trigger procedure argument. You can have more than one trigger on
+a table if you need multiple lo columns in the same table, but don't forget to
+give a different name to each trigger.
-Issues
-* dropping a table will still orphan any objects it contains, as the trigger
- is not actioned.
+Issues
- For now, precede the 'drop table' with 'delete from {table}'. However, this
- could be fixed by having 'drop table' perform an additional
+* Dropping a table will still orphan any objects it contains, as the trigger
+ is not executed.
- 'select lo_unlink({colname}::oid) from {tablename}'
+ Avoid this by preceding the 'drop table' with 'delete from {table}'.
- for each column, before actually dropping the table.
+ If you already have, or suspect you have, orphaned large objects, see
+ the contrib/vacuumlo module to help you clean them up. It's a good idea
+ to run contrib/vacuumlo occasionally as a back-stop to the lo_manage
+ trigger.
* Some frontends may create their own tables, and will not create the
associated trigger(s). Also, users may not remember (or know) to create
the triggers.
- This can be solved, but would involve changes to the parser.
-
As the ODBC driver needs a permanent lo type (& JDBC could be optimised to
use it if it's Oid is fixed), and as the above issues can only be fixed by
some internal changes, I feel it should become a permanent built-in type.
I'm releasing this into contrib, just to get it out, and tested.
Peter Mount <peter@retep.org.uk> June 13 1998
-
/*
- * PostgreSQL type definitions for managed LargeObjects.
+ * PostgreSQL definitions for managed Large Objects.
*
- * $PostgreSQL: pgsql/contrib/lo/lo.c,v 1.14 2003/11/29 19:51:35 pgsql Exp $
+ * $PostgreSQL: pgsql/contrib/lo/lo.c,v 1.15 2005/06/23 00:06:37 tgl Exp $
*
*/
#define atooid(x) ((Oid) strtoul((x), NULL, 10))
-/*
- * This is the internal storage format for managed large objects
- *
- */
-
-typedef Oid Blob;
-
-/*
- * Various forward declarations:
- */
-
-Blob *lo_in(char *str); /* Create from String */
-char *lo_out(Blob * addr); /* Output oid as String */
-Oid lo_oid(Blob * addr); /* Return oid as an oid */
-Blob *lo(Oid oid); /* Return Blob based on oid */
-Datum lo_manage(PG_FUNCTION_ARGS); /* Trigger handler */
-
-/*
- * This creates a large object, and sets its OID to the value in the
- * supplied string.
- *
- * If the string is empty, then a new LargeObject is created, and its oid
- * is placed in the resulting lo.
- */
-Blob *
-lo_in(char *str)
-{
- Blob *result;
- Oid oid;
- int count;
-
- if (strlen(str) > 0)
- {
- count = sscanf(str, "%u", &oid);
-
- if (count < 1)
- ereport(ERROR,
- (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
- errmsg("error in parsing \"%s\"", str)));
-
- if (oid == InvalidOid)
- ereport(ERROR,
- (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
- errmsg("illegal oid: \"%s\"", str)));
- }
- else
- {
- /*
- * There is no Oid passed, so create a new one
- */
- oid = DatumGetObjectId(DirectFunctionCall1(lo_creat,
- Int32GetDatum(INV_READ | INV_WRITE)));
- if (oid == InvalidOid)
- /* internal error */
- elog(ERROR, "InvalidOid returned from lo_creat");
- }
-
- result = (Blob *) palloc(sizeof(Blob));
-
- *result = oid;
-
- return (result);
-}
-
-/*
- * This simply outputs the Oid of the Blob as a string.
- */
-char *
-lo_out(Blob * addr)
-{
- char *result;
-
- if (addr == NULL)
- return (NULL);
-
- result = (char *) palloc(32);
- snprintf(result, 32, "%u", *addr);
- return (result);
-}
-
-/*
- * This function converts Blob to oid.
- *
- * eg: select lo_export(raster::oid,'/path/file') from table;
- *
- */
-Oid
-lo_oid(Blob * addr)
-{
- if (addr == NULL)
- return InvalidOid;
- return (Oid) (*addr);
-}
-
-/*
- * This function is used so we can convert oid's to lo's
- *
- * ie: insert into table values(lo_import('/path/file')::lo);
- *
- */
-Blob *
-lo(Oid oid)
-{
- Blob *result = (Blob *) palloc(sizeof(Blob));
+/* forward declarations */
+Datum lo_manage(PG_FUNCTION_ARGS);
- *result = oid;
- return (result);
-}
/*
- * This handles the trigger that protects us from orphaned large objects
+ * This is the trigger that protects us from orphaned large objects
*/
PG_FUNCTION_INFO_V1(lo_manage);
TupleDesc tupdesc; /* Tuple Descriptor */
HeapTuple rettuple; /* Tuple to be returned */
bool isdelete; /* are we deleting? */
- HeapTuple newtuple = NULL; /* The new value for tuple */
+ HeapTuple newtuple; /* The new value for tuple */
HeapTuple trigtuple; /* The original value of tuple */
- if (!CALLED_AS_TRIGGER(fcinfo))
- /* internal error */
+ if (!CALLED_AS_TRIGGER(fcinfo)) /* internal error */
elog(ERROR, "not fired by trigger manager");
/*
/* Are we deleting the row? */
isdelete = TRIGGER_FIRED_BY_DELETE(trigdata->tg_event);
- /* Get the column were interested in */
+ /* Get the column we're interested in */
attnum = SPI_fnumber(tupdesc, args[0]);
+ if (attnum <= 0)
+ elog(ERROR, "column \"%s\" does not exist", args[0]);
+
/*
* Handle updates
*
char *orig = SPI_getvalue(trigtuple, tupdesc, attnum);
char *newv = SPI_getvalue(newtuple, tupdesc, attnum);
- if (orig != NULL && (newv == NULL || strcmp(orig, newv)))
+ if (orig != NULL && (newv == NULL || strcmp(orig, newv) != 0))
DirectFunctionCall1(lo_unlink,
ObjectIdGetDatum(atooid(orig)));
* Handle deleting of rows
*
* Here, we unlink the large object associated with the managed attribute
- *
*/
if (isdelete)
{
--
--- PostgreSQL code for LargeObjects
+-- PostgreSQL code for managed Large Objects
--
--- $PostgreSQL: pgsql/contrib/lo/lo.sql.in,v 1.12 2005/01/29 22:35:02 tgl Exp $
---
---
--- Create the data type
+-- $PostgreSQL: pgsql/contrib/lo/lo.sql.in,v 1.13 2005/06/23 00:06:37 tgl Exp $
--
--- used by the lo type, it takes an oid and returns an lo object
-
-- Adjust this setting to control where the objects get created.
SET search_path = public;
-CREATE FUNCTION lo_in(cstring)
-RETURNS lo
-AS 'MODULE_PATHNAME'
-LANGUAGE C IMMUTABLE STRICT;
-
--- used by the lo type, it returns the oid of the object
-CREATE FUNCTION lo_out(lo)
-RETURNS cstring
-AS 'MODULE_PATHNAME'
-LANGUAGE C IMMUTABLE STRICT;
-
--- finally the type itself
-CREATE TYPE lo (
- INTERNALLENGTH = 4,
- EXTERNALLENGTH = variable,
- INPUT = lo_in,
- OUTPUT = lo_out
-);
-
--- this returns the oid associated with a lo object
-CREATE FUNCTION lo_oid(lo)
-RETURNS oid
-AS 'MODULE_PATHNAME'
-LANGUAGE C IMMUTABLE STRICT;
+--
+-- Create the data type ... now just a domain over OID
+--
--- same function, named to allow it to be used as a type coercion, eg:
--- CREATE TABLE a (image lo);
--- SELECT image::oid FROM a;
---
-CREATE FUNCTION oid(lo)
-RETURNS oid
-AS 'MODULE_PATHNAME', 'lo_oid'
-LANGUAGE C IMMUTABLE STRICT;
-CREATE CAST (lo as oid) WITH FUNCTION oid(lo) AS IMPLICIT;
+CREATE DOMAIN lo AS pg_catalog.oid;
--- this allows us to convert an oid to a managed lo object
--- ie: insert into test values (lo_import('/fullpath/file')::lo);
-CREATE FUNCTION lo(oid)
-RETURNS lo
-AS 'MODULE_PATHNAME'
-LANGUAGE C IMMUTABLE STRICT;
-CREATE CAST (oid as lo) WITH FUNCTION lo(oid) AS IMPLICIT;
+--
+-- For backwards compatibility, define a function named lo_oid.
+--
+-- The other functions that formerly existed are not needed because
+-- the implicit casts between a domain and its underlying type handle them.
+--
+CREATE FUNCTION lo_oid(lo) RETURNS pg_catalog.oid AS
+'SELECT $1::pg_catalog.oid' LANGUAGE SQL STRICT IMMUTABLE;
-- This is used in triggers
CREATE FUNCTION lo_manage()
-RETURNS trigger
+RETURNS pg_catalog.trigger
AS 'MODULE_PATHNAME'
LANGUAGE C;
--
--- This removes the type (and a test table)
+-- This removes the LO type
-- It's used just for development
--
-- Adjust this setting to control where the objects get created.
SET search_path = public;
--- remove our test table
-DROP TABLE a;
-
--- now drop the type and associated C functions
+-- drop the type and associated functions
DROP TYPE lo CASCADE;
-- the trigger function has no dependency on the type, so drop separately
--
-- It's used just for development
--
+-- XXX would be nice to turn this into a proper regression test
+--
+
+-- Check what is in pg_largeobject
+SELECT count(DISTINCT loid) FROM pg_largeobject;
-- ignore any errors here - simply drop the table if it already exists
DROP TABLE a;
CREATE TABLE a (fname name,image lo);
-- insert a null object
-INSERT INTO a VALUES ('null');
-
--- insert an empty large object
-INSERT INTO a VALUES ('empty','');
+INSERT INTO a VALUES ('empty');
-- insert a large object based on a file
-INSERT INTO a VALUES ('/etc/group',lo_import('/etc/group')::lo);
+INSERT INTO a VALUES ('/etc/group', lo_import('/etc/group')::lo);
-- now select the table
SELECT * FROM a;
--- this select also returns an oid based on the lo column
+-- check that coercion to plain oid works
SELECT *,image::oid from a;
-- now test the trigger
EXECUTE PROCEDURE lo_manage(image);
-- insert
-INSERT INTO a VALUES ('aa','');
+INSERT INTO a VALUES ('aa', lo_import('/etc/hosts'));
SELECT * FROM a
WHERE fname LIKE 'aa%';
WHERE fname LIKE 'aa%';
-- update the 'empty' row which should be null
-UPDATE a SET image=lo_import('/etc/hosts')::lo
+UPDATE a SET image=lo_import('/etc/hosts')
WHERE fname='empty';
SELECT * FROM a
WHERE fname LIKE 'empty%';
-- This deletes the table contents. Note, if you comment this out, and
-- expect the drop table to remove the objects, think again. The trigger
--- doesn't get thrown by drop table.
+-- doesn't get fired by drop table.
DELETE FROM a;
-- finally drop the table
DROP TABLE a;
+-- Check what is in pg_largeobject ... if different from original, trouble
+SELECT count(DISTINCT loid) FROM pg_largeobject;
+
-- end of tests
-$PostgreSQL: pgsql/contrib/vacuumlo/README.vacuumlo,v 1.4 2003/11/29 19:51:36 pgsql Exp $
+$PostgreSQL: pgsql/contrib/vacuumlo/README.vacuumlo,v 1.5 2005/06/23 00:06:37 tgl Exp $
This is a simple utility that will remove any orphaned large objects out of a
PostgreSQL database. An orphaned LO is considered to be any LO whose OID
does not appear in any OID data column of the database.
+If you use this, you may also be interested in the lo_manage trigger in
+contrib/lo. lo_manage is useful to try to avoid creating orphaned LOs
+in the first place.
+
Compiling
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