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DECLARE(7) PostgreSQL 9.5.0 Documentation DECLARE(7)
NAME
DECLARE - define a cursor
SYNOPSIS
DECLARE name [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
CURSOR [ { WITH | WITHOUT } HOLD ] FOR query
DESCRIPTION
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(7).
Note
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 40.7, "Cursors", in the documentation.
PARAMETERS
name
The name of the cursor to be created.
BINARY
Causes the cursor to return data in binary rather than in text
format.
INSENSITIVE
Indicates 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 PostgreSQL, this is the default behavior;
so this key word has no effect and is only accepted for
compatibility with the SQL standard.
SCROLL
NO SCROLL
SCROLL 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 for
details.
WITH HOLD
WITHOUT HOLD
WITH 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.
query
A SELECT(7) or VALUES(7) command which will provide the rows to be
returned by the cursor.
The key words BINARY, INSENSITIVE, and SCROLL can appear in any order.
NOTES
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
psql, are not prepared to handle binary cursors and expect data to come
back in the text format.
Note
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 PostgreSQL reports
an error if such a command is used outside a transaction block. Use
BEGIN(7) and COMMIT(7) (or ROLLBACK(7)) 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, PostgreSQL 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.
Caution
Scrollable and WITH HOLD cursors may give unexpected results if
they invoke any volatile functions (see Section 35.6, "Function
Volatility Categories", in the documentation). 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(7) 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.)
Caution
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.
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
PostgreSQL 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 PostgreSQL, 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.
EXAMPLES
To declare a cursor:
DECLARE liahona CURSOR FOR SELECT * FROM films;
See FETCH(7) for more examples of cursor usage.
COMPATIBILITY
The SQL standard says that it is implementation-dependent whether
cursors are sensitive to concurrent updates of the underlying data by
default. In PostgreSQL, 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.
PostgreSQL permits cursors to be used interactively.
Binary cursors are a PostgreSQL extension.
SEE ALSO
CLOSE(7), FETCH(7), MOVE(7)
PostgreSQL 9.5.0 2016 DECLARE(7)
OPENSSL_LH_COMPFUNC(3) OpenSSL OPENSSL_LH_COMPFUNC(3)
NAME
LHASH, DECLARE_LHASH_OF, OPENSSL_LH_COMPFUNC, OPENSSL_LH_HASHFUNC,
OPENSSL_LH_DOALL_FUNC, LHASH_DOALL_ARG_FN_TYPE,
IMPLEMENT_LHASH_HASH_FN, IMPLEMENT_LHASH_COMP_FN, lh_TYPE_new,
lh_TYPE_free, lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve,
lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_error - dynamic hash table
SYNOPSIS
#include <openssl/lhash.h>
DECLARE_LHASH_OF(TYPE);
LHASH *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
void lh_TYPE_free(LHASH_OF(TYPE) *table);
TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data);
TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data);
TYPE *lh_TYPE_retrieve(LHASH_OF(TYPE) *table, TYPE *data);
void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
TYPE *arg);
int lh_TYPE_error(LHASH_OF(TYPE) *table);
typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
DESCRIPTION
This library implements type-checked dynamic hash tables. The hash
table entries can be arbitrary structures. Usually they consist of key
and value fields. In the description here, TYPE is used a placeholder
for any of the OpenSSL datatypes, such as SSL_SESSION.
lh_TYPE_new() creates a new LHASH_OF(TYPE) structure to store arbitrary
data entries, and specifies the 'hash' and 'compare' callbacks to be
used in organising the table's entries. The hash callback takes a
pointer to a table entry as its argument and returns an unsigned long
hash value for its key field. The hash value is normally truncated to
a power of 2, so make sure that your hash function returns well mixed
low order bits. The compare callback takes two arguments (pointers to
two hash table entries), and returns 0 if their keys are equal, nonzero
otherwise.
If your hash table will contain items of some particular type and the
hash and compare callbacks hash/compare these types, then the
IMPLEMENT_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros can be used
to create callback wrappers of the prototypes required by lh_TYPE_new()
as shown in this example:
/*
* Implement the hash and compare functions; "stuff" can be any word.
*/
static unsigned long stuff_hash(const TYPE *a)
{
...
}
static int stuff_cmp(const TYPE *a, const TYPE *b)
{
...
}
/*
* Implement the wrapper functions.
*/
static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)
If the type is going to be used in several places, the following macros
can be used in a common header file to declare the function wrappers:
DECLARE_LHASH_HASH_FN(stuff, TYPE)
DECLARE_LHASH_COMP_FN(stuff, TYPE)
Then a hash table of TYPE objects can be created using this:
LHASH_OF(TYPE) *htable;
htable = lh_TYPE_new(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
lh_TYPE_free() frees the LHASH_OF(TYPE) structure table. Allocated hash
table entries will not be freed; consider using lh_TYPE_doall() to
deallocate any remaining entries in the hash table (see below).
lh_TYPE_insert() inserts the structure pointed to by data into table.
If there already is an entry with the same key, the old value is
replaced. Note that lh_TYPE_insert() stores pointers, the data are not
copied.
lh_TYPE_delete() deletes an entry from table.
lh_TYPE_retrieve() looks up an entry in table. Normally, data is a
structure with the key field(s) set; the function will return a pointer
to a fully populated structure.
lh_TYPE_doall() will, for every entry in the hash table, call func with
the data item as its parameter. For example:
/* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
void TYPE_cleanup_doall(TYPE *a);
/* Implement a prototype-compatible wrapper for "TYPE_cleanup" */
IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)
/* Call "TYPE_cleanup" against all items in a hash table. */
lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));
/* Then the hash table itself can be deallocated */
lh_TYPE_free(hashtable);
When doing this, be careful if you delete entries from the hash table
in your callbacks: the table may decrease in size, moving the item that
you are currently on down lower in the hash table - this could cause
some entries to be skipped during the iteration. The second best
solution to this problem is to set hash->down_load=0 before you start
(which will stop the hash table ever decreasing in size). The best
solution is probably to avoid deleting items from the hash table inside
a "doall" callback!
lh_TYPE_doall_arg() is the same as lh_TYPE_doall() except that func
will be called with arg as the second argument and func should be of
type LHASH_DOALL_ARG_FN_TYPE (a callback prototype that is passed both
the table entry and an extra argument). As with lh_doall(), you can
instead choose to declare your callback with a prototype matching the
types you are dealing with and use the declare/implement macros to
create compatible wrappers that cast variables before calling your
type-specific callbacks. An example of this is demonstrated here
(printing all hash table entries to a BIO that is provided by the
caller):
/* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
/* Implement a prototype-compatible wrapper for "TYPE_print" */
static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)
/* Print out the entire hashtable to a particular BIO */
lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
logging_bio);
lh_TYPE_error() can be used to determine if an error occurred in the
last operation.
RETURN VALUES
lh_TYPE_new() returns NULL on error, otherwise a pointer to the new
LHASH structure.
When a hash table entry is replaced, lh_TYPE_insert() returns the value
being replaced. NULL is returned on normal operation and on error.
lh_TYPE_delete() returns the entry being deleted. NULL is returned if
there is no such value in the hash table.
lh_TYPE_retrieve() returns the hash table entry if it has been found,
NULL otherwise.
lh_TYPE_error() returns 1 if an error occurred in the last operation, 0
otherwise. It's meaningful only after non-retrieve operations.
lh_TYPE_free(), lh_TYPE_doall() and lh_TYPE_doall_arg() return no
values.
NOTE
The LHASH code is not thread safe. All updating operations, as well as
lh_TYPE_error call must be performed under a write lock. All retrieve
operations should be performed under a read lock, unless accurate usage
statistics are desired. In which case, a write lock should be used for
retrieve operations as well. For output of the usage statistics, using
the functions from OPENSSL_LH_stats(3), a read lock suffices.
The LHASH code regards table entries as constant data. As such, it
internally represents lh_insert()'d items with a "const void *" pointer
type. This is why callbacks such as those used by lh_doall() and
lh_doall_arg() declare their prototypes with "const", even for the
parameters that pass back the table items' data pointers - for
consistency, user-provided data is "const" at all times as far as the
LHASH code is concerned. However, as callers are themselves providing
these pointers, they can choose whether they too should be treating all
such parameters as constant.
As an example, a hash table may be maintained by code that, for reasons
of encapsulation, has only "const" access to the data being indexed in
the hash table (i.e. it is returned as "const" from elsewhere in their
code) - in this case the LHASH prototypes are appropriate as-is.
Conversely, if the caller is responsible for the life-time of the data
in question, then they may well wish to make modifications to table
item passed back in the lh_doall() or lh_doall_arg() callbacks (see the
"TYPE_cleanup" example above). If so, the caller can either cast the
"const" away (if they're providing the raw callbacks themselves) or use
the macros to declare/implement the wrapper functions without "const"
types.
Callers that only have "const" access to data they're indexing in a
table, yet declare callbacks without constant types (or cast the
"const" away themselves), are therefore creating their own risks/bugs
without being encouraged to do so by the API. On a related note, those
auditing code should pay special attention to any instances of
DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
without any "const" qualifiers.
BUGS
lh_TYPE_insert() returns NULL both for success and error.
SEE ALSO
OPENSSL_LH_stats(3)
HISTORY
In OpenSSL 1.0.0, the lhash interface was revamped for better type
checking.
COPYRIGHT
Copyright 2000-2022 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
1.1.1v 2023-08-01 OPENSSL_LH_COMPFUNC(3)