The pgExpress Driver allows using of functions as stored procedures.
An usage example:
Example 2.4. Stored Procedures usage
procedure TForm1.Button1Click(Sender: TObject); begin with SQLDataset1 do begin Close; CommandType := ctQuery; CommandText := 'create or replace function Test1(int2, int2) returns' + ' int2 as ''select $1 + $2;'' language ''SQL'';'; ExecSQL; CommandType := ctStoredProc; // This MUST be before setting paramaters. // VCL clears Params on setting CommandText. CommandText := 'Test1'; Params.Value := 10; Params.Value := 20; ExecSQL; ShowMessage(Params.AsString); // Will display 30 end; end;
PostgreSQL supports a feature named “Fetch Cursors”. These special cursors allow the records to be retrieved from the server in smaller sets then the defaulbehaviort, which is retrieving the full recordset. To achieve this, the the PostgreSQL DECLARE and FETCH commands are used. This can accelerate the queries response in some situations, specially when the connection to the server is fast and the recordset is large. Also, Fetch Cursors do not need the whole query to be processed: they can start retrieving data as soon as the requested number of rows is available. Finally, Fetch Cursors will consume less memory then regular cursors because fewer rows stays in memory each time.
As of version 1.60, the pgExpress Driver features automatic
FETCH cursors. The only thing needed is setting the special param
BlockRead; however, you probably will want to
change also the
RowsetSize param (please check
your Kylix/Delphi documentation) if you want (as of Delphi 6/7 and
Kylix 1/2/3, default VCL/CLX
Internally, the pgExpress Driver will DECLARE a cursor with a unique name (per transaction) for each dataset and use FETCH commands as their rows are requested. When the dataset is freed, it's cursor will be closed.
The number of rows retrieved on each internal fetch operation is given by the
RowsetSize param. A negative
RowsetSize param will issue a
FETCH ALL command,
what actually does not differ much then a regular query without use of a FETCH command:
all rows will be retrieved at once.
Usually larger values, like
100 (or even more, depending on your environment),
would give a better performance. Please try and see what value is best for your
application, but have in mind that each FETCH command means a
separated query and that could slow down dataset scrolling over slower
dbxconnections(.ini) setup would be:
[PGEConnection] BlobSize=32 Database=localhost/mydb,BlockRead=True DriverName=PostgreSQL Password=temp123 User_Name=foo RowsetSize=200
An additional, good side effect of using Fetch Cursors is that the overall memory used is much less that if you retrieve all rows at once, due to libpq caching. For large datasets, using this setting is a smart option. Please check Q: 7 for more details.
TClientDataset.FetchOnDemandto True. If you don't set
True, Fetch Cursors will be worthless because the
TClientDatasetwill be caching all rows in memory. Of course when not using a caching grid such as
TClientDataset, this is not needed.
RetainCursorSpecial Parameter will be ignored if Fetch Cursors are in use. For more details on the inners of Fetch Cursors, please refer to the follow PostgreSQL documentation links:
TSQLConnection.StartTransactionbefore opening the cursor.
As of PostgreSQL 7.3, three authentication methods are available:
Since the pgExpress Driver is
libpq based, use these three methods are automatic,
and will follow what you have defined on the
pg_hba.conf file (please check
We advice using the
MD5 method (PostgreSQL
7.2 and above), unless you are using
SSL or other
encryption wrapper in your connection to the server.
The pgExpress Driver has the ability to automatically detect the numeric format used on you server; namely, the Decimal and the thousands separators. There should be no need to set numeric formats manually, but in case you want or need, try the ServerDecimalSeparator special parameter.
In case the automatic detection do not work for you, please mail us at firstname.lastname@example.org
Since dbExpress (pre-protocol 3.0) does not support int8 types natively, the 1.X series of the pgExpress Driver used to map them as TBcdField fields by default (AsBcd).
As of pgExpress Driver v2.0+, int8 fields are mapped
TLargeintField fields using an internal hack.
However, that hack will not work on all situations;
specially if you need to write to int8 fields, map them as
type TBcdField using the AsBcd special param
and everything will work as you expect on the Delphi/Kylix side.
If you, however, need only read-only access to the int8 fields, you can use that hack and enjoy a native Int8 suppport instead of mapping to another type.
Note that Int8 fields support is still not official in dbExpress as of this is written.
While they will work most of time, the
TIndexDefs create on these fields will
not be correctly handled by VCL. You can get "Key violation" messages when
trying to insert new values in this field, even if the values differ. To prevent this, you can try any
of the following workarounds:
TSQLDataset. In older versions then D7, it's the same as enabling NoMetadata.
IndexDefgenerated automatically by dbExpress. After opening your
ClientDataset1.IndexDefs.Delete(0)for removing that
IndexDef. However, this will make the field unindexed; the exception will not be raised even when you insert duplicate values in the field.
The lack of a
First() interface for dbExpress
cursors make it necessary to run any query twice if you want to access a previous record again; the
dbExpress technology is currently forward-only. Of course, Client Datasets
TCustomCachedDataset descendants) will cache records so random access is
possible; but non-cached SQL Datasets (
TCustomSQLDataset descendants) will make the query
The pgExpress Driver implements an experimental setting, RetainCursor, that will make non-cached datatasets access
MUCH faster to access from the second time, since the records will be retrieved from the internal
libpq cache instead of being retrieved once again from the server. Basically, if RetainCursor is True, the query is a select query and the query is the same as
the last one executed, the results displayed will be the same from the cache.
This behavior is controlled by the RetainCursor Special Parameter.
If you are using RetainCursor = True and want a particular query to be
trully executed again, instead of retrieved from the cache, add a '
!' char to the
beginning of the query (it will be automatically stripped by the pgExpress Driver):
!select * from pg_type;
As of pgExpress v3.50, there are three kinds of BLOBs supported, according to their mappings:
Detailed explain of these modes follows below. For very large streams, the best approach is to use Section 6.7.1, “Large Objects (BLOBs)”; however, for small (around 1-2Mb or even more), such as pictures, it's much easier and cleaner to use Bytea fields as BLOBs.
Large Objects implement file-like I/O operations on PostgreSQL through a complicated API. The pgExpress Driver allows using them automatically through the regular dbExpress API.
As of pgExpress v2.31, we had to make some changes to the way pgExpress handles Large Objects and also changes to this documentation. Due to a bug in PostgreSQL up to v7.4 (PQftype() won't return the current oid for a domain, but for it's defined type instead) the way to create the 'lo' type has changed as shown below; it used to be:
create domain lo oid
but that will not work as it should due to the way the backend protocol works.
If you have a production database using the 'lo' type/domain declared in a different way then below, we suggest to pg_dump the database structure and data separately, then change the struct dump script to create the 'lo' type as shown below, and finally recreate the database structure with the new 'lo' type definition and restore the data.
The Large Object 'lo' field type isn't declared by the default in the PostgreSQL distribution, so
we must do it manually. However, as of PostgreSQL 8.0,
Windows users used
to install the contrib modules from the installer
might already have installed the 'lo' contrib module, which adds a compatible lo type,
and these steps will not be necessary. Of course, users from other platforms that installed that
contrib module will also have that type automatically created.
So, before creating the lo type, check if it isn't already created:
howe=# select oid, typname from pg_type where typname='lo'; oid | typname -------+--------- 17245 | lo (1 row)
CREATE FUNCTION lo_in(cstring) RETURNS lo AS 'int4in' LANGUAGE 'internal' WITH (ISCACHABLE, ISSTRICT); CREATE FUNCTION lo_out(lo) RETURNS cstring AS 'int4out' LANGUAGE 'internal' WITH (ISCACHABLE, ISSTRICT); CREATE TYPE lo( internallength = 4, externallength=10, input = lo_in, output = lo_out, alignment = int4, default = '', passedbyvalue ); CREATE CAST (lo AS oid) WITHOUT FUNCTION;
...or (PostgreSQL 7.2X and below):
create type lo( internallength=4, externallength=10, input=oidin, output=oidout, default='', passedbyvalue );
You can then use this type normally on your tables:
Example 2.6. Creating a table with a Large Object
create table employee(id integer, name varchar(30), picture lo);
The pgExpress Driver can read Large Object (BLOB) fields without problems; however, due to the way the dbExpress technology was designed and the particular implementation of Large Objects on PostgreSQL, the pgExpress Driver has problems on updating Large Object fields.
PostgreSQL refers to Large Objects using a OID that
points to the the real data.The
libpq library needs this OID to do all sort of operations on the
Large Object fields. The problem is that the dbExpress API will be expecting
only the BLOB field's data; the OID information has no way to be stored. After the BLOB
field is processed internally by the VCL/CLX and is sent back to the pgExpress Driver for being stored in
the database, the original OID of the Large Object can't be retrieved, so pgExpress won't know which BLOB is refers to.
This means that we can't alter the original Large Object, and if we create a new Large Object, the
original Large Object will end up with as a orphan Large Object (a LO that exist but is not referenced
by any rows, wasting disk space), unless of course another row refers to the same LO, what is not a common situation.
At Vita Voom Software™, we understand that Large Objects support it is crucial for some users and thus we have the following suggestion as a workaround to this problem, which affects also the JDBC and ODBC drivers:
lo_clean()function above could easily be modified to reuse the old OID value in the case of a UPDATE query, by adding an statment such as:
update pg_largeobject set loid=old.oid where loid=new.oid
contrib/lodirectory on the PostgreSQL distributions contains code that helps avoiding orphan Large Objects; you might be insterested in using it, or even reading the docs for more background on the subject.
Create your table normally, including the BLOB field (please read this section about BLOB fields declaration):
create table lo_test(id serial, image lo);
create or replace function test_lo_clean() returns trigger as ' declare lo_oid oid; begin -- If it is an update action but the BLOB (lo) field was not changed, dont do anything if (TG_OP = ''UPDATE'') then if (old.image = new.image) or (old.image is null) then return new; end if; end if; select into lo_oid loid from pg_largeobject where lo_oid = oid(old.image); if found then perform lo_unlink(lo_oid); end if; return new; end' language 'plpgsql';
create trigger lo_cleanup after delete or update on lo_test -- must be after to avoid deleting the Large Object if record is not deleted for each row execute procedure test_lo_clean();
The function above uses the pl/PgSql language, which must be created in the datbase if it wasn't already. This can be accomplished easily through the following command (at the shell prompt):
$ createlang -d dbname plpgsql
For more details on creating languages, please check here.
This trigger will avoid that orphan Large Objects are left on the table. Note that triggers are fast; in fact, referential integrity is implemented internally in PostgreSQL using them. If you want to try another code to do it, feel free; all you need to do is ensure no orphan OIDs are left on the pg_largeobject table.
You might have to make some small changes to the
test_lo_clean() function to reflect
your actual table and field names, if you have large objects reused in more then one field/record, etc.
Hopefully the PostgreSQL developers will introduce a new API for handling Large Objects that does not suffer from these limitations.
select lo_unlink(lo_column) from mytable;
As of pgExpress 3.50, it is possible (and actually the new default behavior) to map
bytea fields as
ftBinary) through using theByteaMode Special Param.
This makes much easier to use BLOB fields on pgExpress since there is no need
to use the Large Objects API.
While this is activated by default, it could conflict with older datasets that are mapping bytea
TVarBytes fields, which was the old pgExpress behaviour. To force mapping
bytea fields as
TVarBytesField again, set the special param
ByteaMode Special Param to
asVarBytes. To completely disable
Section 6.7.1, “Large Objects (BLOBs)” support by mapping bytea fields as
ftBinary, set the special param
ByteaMode Special Param
Remember that bytea chars greater that 128 must be quoted as octals (ex: \343), what will make streams be greater in size (4x more for those chars that have to be quoted) and also waste more bandwidth - a mean of about 2.5x for a random stream. For small streams, this should not be a problem; however, for greater streams, consider using Section 6.7.1, “Large Objects (BLOBs)”. This protocol behavior is internal to PostgreSQL and the pgExpress Driver must conform to it.
When assigning a value to a bytea
is mapped as ftGraphic, you'll tipically
use the TParam.asBlob property, or the
After doing it, do not forget to set the TParam.DataType to
ftGraphic, because these methods set TParam.DataType to True;
otherwise, the fields will be try to be saved as Large Objects, and
that would be the source of weird errors. Example:
with Dataset1.ParamByName('value') do begin ParamType := ptInput; AsBlob := aData; DataType := ftGraphic; end;
Again, this is only needed by bytea fields mapped as
ftGraphic, on the specific conditions mentioned above.
To allow easy use of TEXT fields, the ppgExpress Driver maps them as
ftMemo by default; they can have arbitrary lengths,
contrary to regular string (
ftString) fields, whose must have a specific length.
When assigning a value to a
TParam, use always the
TParam.asMemo property (and not TParam.asBlob, which
is intented to work with binary blobs(subtype
PostgreSQL has built-in support for SSL. However, on the backend (client) side, we depend on
support for actually encrypting the connection. Make sure your
libpq is SSL-enabled iof you want to make
There are, however other ways to do it, even if your libpq is not SSL-enabled. The simplest is making a tunnel using either SSH or Stunnel. Please go to the referred site for more details.
PostgreSQL by default will print NOTICE messages (generated by the server and inside functions by statements such as
RAISE NOTICE) in the server's standard output. The pgExpress Driver implements a hook which allows those
messages to be logged along other regular messages so that a
TSQLMonitor component will be able to
log them all automatically.
The pgExpress Driver has asynchronous connections support. Their advantage is that a query could be cancelled while in execution in the server. Other then that, there is no real advantage on then.
To activate asynchronous support on pgExpress, set this on your dbxconnections.ini file (or equivalent):
To force synchronous connections, whose are more compatible in general, use:
If you have problems with async support, please switch to synchronous mode.
Whenever you send a query to the PostgreSQL server, it can delay a long time to return a result to your
application. Typical cases include a very long resultset, lots of computations/calculations, or even a
function hangup/infinite loop. It is nice to let you user cancel a query if they want to, thus returning control
program, and quitting things nicely in the server side. The way to accomplish this is by using
asynchronous connections, and that can be cancelled by calling the
The pgExpress Driver wraps that API nicely in the pgeDriverUtils.CancelQuery function. Calling it will make the server try its best to cancel the currently query and return control to the client application. This has the same effect as pressing Ctrl+C in the psql console.