DECLARE — define a cursor
DECLAREname[ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ] CURSOR [ { WITH | WITHOUT } HOLD ] FORquery
DECLARE allows a user to create cursors, which
can be used to retrieve
a small number of rows at a time out of a larger query.
After the cursor is created, rows are fetched from it using
FETCH.
This page describes usage of cursors at the SQL command level. If you are trying to use cursors inside a PL/pgSQL function, the rules are different — see Section 41.7.
nameThe name of the cursor to be created.
BINARYCauses the cursor to return data in binary rather than in text format.
INSENSITIVEIndicates that data retrieved from the cursor should be unaffected by updates to the table(s) underlying the cursor that occur after the cursor is created. In LightDB, this is the default behavior; so this key word has no effect and is only accepted for compatibility with the SQL standard.
SCROLLNO SCROLLSCROLL specifies that the cursor can be used
to retrieve rows in a nonsequential fashion (e.g.,
backward). Depending upon the complexity of the query's
execution plan, specifying SCROLL might impose
a performance penalty on the query's execution time.
NO SCROLL specifies that the cursor cannot be
used to retrieve rows in a nonsequential fashion. The default is to
allow scrolling in some cases; this is not the same as specifying
SCROLL. See Notes
below for details.
WITH HOLDWITHOUT HOLDWITH HOLD specifies that the cursor can
continue to be used after the transaction that created it
successfully commits. WITHOUT HOLD specifies
that the cursor cannot be used outside of the transaction that
created it. If neither WITHOUT HOLD nor
WITH HOLD is specified, WITHOUT
HOLD is the default.
queryA SELECT or VALUES command which will provide the rows to be returned by the cursor.
The key words BINARY,
INSENSITIVE, and SCROLL can
appear in any order.
Normal cursors return data in text format, the same as a
SELECT would produce. The BINARY option
specifies that the cursor should return data in binary format.
This reduces conversion effort for both the server and client,
at the cost of more programmer effort to deal with platform-dependent
binary data formats.
As an example, if a query returns a value of one from an integer column,
you would get a string of 1 with a default cursor,
whereas with a binary cursor you would get
a 4-byte field containing the internal representation of the value
(in big-endian byte order).
Binary cursors should be used carefully. Many applications, including ltsql, are not prepared to handle binary cursors and expect data to come back in the text format.
When the client application uses the “extended query” protocol
to issue a FETCH command, the Bind protocol message
specifies whether data is to be retrieved in text or binary format.
This choice overrides the way that the cursor is defined. The concept
of a binary cursor as such is thus obsolete when using extended query
protocol — any cursor can be treated as either text or binary.
Unless WITH HOLD is specified, the cursor
created by this command can only be used within the current
transaction. Thus, DECLARE without WITH
HOLD is useless outside a transaction block: the cursor would
survive only to the completion of the statement. Therefore
LightDB reports an error if such a
command is used outside a transaction block.
Use
BEGIN and
COMMIT
(or ROLLBACK)
to define a transaction block.
If WITH HOLD is specified and the transaction
that created the cursor successfully commits, the cursor can
continue to be accessed by subsequent transactions in the same
session. (But if the creating transaction is aborted, the cursor
is removed.) A cursor created with WITH HOLD
is closed when an explicit CLOSE command is
issued on it, or the session ends. In the current implementation,
the rows represented by a held cursor are copied into a temporary
file or memory area so that they remain available for subsequent
transactions.
WITH HOLD may not be specified when the query
includes FOR UPDATE or FOR SHARE.
The SCROLL option should be specified when defining a
cursor that will be used to fetch backwards. This is required by
the SQL standard. However, for compatibility with earlier
versions, LightDB will allow
backward fetches without SCROLL, if the cursor's query
plan is simple enough that no extra overhead is needed to support
it. However, application developers are advised not to rely on
using backward fetches from a cursor that has not been created
with SCROLL. If NO SCROLL is
specified, then backward fetches are disallowed in any case.
Backward fetches are also disallowed when the query
includes FOR UPDATE or FOR SHARE; therefore
SCROLL may not be specified in this case.
Scrollable and WITH HOLD cursors may give unexpected
results if they invoke any volatile functions (see Section 36.7). When a previously fetched row is
re-fetched, the functions might be re-executed, perhaps leading to
results different from the first time. One workaround for such cases
is to declare the cursor WITH HOLD and commit the
transaction before reading any rows from it. This will force the
entire output of the cursor to be materialized in temporary storage,
so that volatile functions are executed exactly once for each row.
If the cursor's query includes FOR UPDATE or FOR
SHARE, then returned rows are locked at the time they are first
fetched, in the same way as for a regular
SELECT command with
these options.
In addition, the returned rows will be the most up-to-date versions;
therefore these options provide the equivalent of what the SQL standard
calls a “sensitive cursor”. (Specifying INSENSITIVE
together with FOR UPDATE or FOR SHARE is an error.)
It is generally recommended to use FOR UPDATE if the cursor
is intended to be used with UPDATE ... WHERE CURRENT OF or
DELETE ... WHERE CURRENT OF. Using FOR UPDATE
prevents other sessions from changing the rows between the time they are
fetched and the time they are updated. Without FOR UPDATE,
a subsequent WHERE CURRENT OF command will have no effect if
the row was changed since the cursor was created.
Another reason to use FOR UPDATE is that without it, a
subsequent WHERE CURRENT OF might fail if the cursor query
does not meet the SQL standard's rules for being “simply
updatable” (in particular, the cursor must reference just one table
and not use grouping or ORDER BY). Cursors
that are not simply updatable might work, or might not, depending on plan
choice details; so in the worst case, an application might work in testing
and then fail in production. If FOR UPDATE is
specified, the cursor is guaranteed to be updatable.
The main reason not to use FOR UPDATE with WHERE
CURRENT OF is if you need the cursor to be scrollable, or to be
insensitive to the subsequent updates (that is, continue to show the old
data). If this is a requirement, pay close heed to the caveats shown
above.
The SQL standard only makes provisions for cursors in embedded
SQL. The LightDB
server does not implement an OPEN statement for
cursors; a cursor is considered to be open when it is declared.
However, ECPG, the embedded SQL
preprocessor for LightDB, supports
the standard SQL cursor conventions, including those involving
DECLARE and OPEN statements.
You can see all available cursors by querying the pg_cursors
system view.
To declare a cursor:
DECLARE liahona CURSOR FOR SELECT * FROM films;
See FETCH for more examples of cursor usage.
The SQL standard says that it is implementation-dependent whether cursors
are sensitive to concurrent updates of the underlying data by default. In
LightDB, cursors are insensitive by default,
and can be made sensitive by specifying FOR UPDATE. Other
products may work differently.
The SQL standard allows cursors only in embedded SQL and in modules. LightDB permits cursors to be used interactively.
Binary cursors are a LightDB extension.