44.3. Built-in Functions

44.3. Built-in Functions
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44.3.1. Database Access from PL/Perl
44.3.2. Utility Functions in PL/Perl

44.3.1. Database Access from PL/Perl

+ Access to the database itself from your Perl function can be done + via the following functions: +

+ spi_exec_query(query [, max-rows]) + +

+ spi_exec_query executes an SQL command and +returns the entire row set as a reference to an array of hash +references. You should only use this command when you know +that the result set will be relatively small. Here is an +example of a query (SELECT command) with the +optional maximum number of rows: + +

+$rv = spi_exec_query('SELECT * FROM my_table', 5);
+

+ This returns up to 5 rows from the table + my_table. If my_table + has a column my_column, you can get that + value from row $i of the result like this: +

+$foo = $rv->{rows}[$i]->{my_column};
+

+ The total number of rows returned from a SELECT + query can be accessed like this: +

+$nrows = $rv->{processed}
+

+ Here is an example using a different command type: +

+$query = "INSERT INTO my_table VALUES (1, 'test')";
+$rv = spi_exec_query($query);
+

+ You can then access the command status (e.g., + SPI_OK_INSERT) like this: +

+$res = $rv->{status};
+

+ To get the number of rows affected, do: +

+$nrows = $rv->{processed};
+

+ Here is a complete example: +

+CREATE TABLE test (
+    i int,
+    v varchar
+);
+
+INSERT INTO test (i, v) VALUES (1, 'first line');
+INSERT INTO test (i, v) VALUES (2, 'second line');
+INSERT INTO test (i, v) VALUES (3, 'third line');
+INSERT INTO test (i, v) VALUES (4, 'immortal');
+
+CREATE OR REPLACE FUNCTION test_munge() RETURNS SETOF test AS $$
+    my $rv = spi_exec_query('select i, v from test;');
+    my $status = $rv->{status};
+    my $nrows = $rv->{processed};
+    foreach my $rn (0 .. $nrows - 1) {
+        my $row = $rv->{rows}[$rn];
+        $row->{i} += 200 if defined($row->{i});
+        $row->{v} =~ tr/A-Za-z/a-zA-Z/ if (defined($row->{v}));
+        return_next($row);
+    }
+    return undef;
+$$ LANGUAGE plperl;
+
+SELECT * FROM test_munge();
+

+ spi_query(command) + + + spi_fetchrow(cursor) + + + spi_cursor_close(cursor) + +

+ spi_query and spi_fetchrow + work together as a pair for row sets which might be large, or for cases + where you wish to return rows as they arrive. + spi_fetchrow works only with + spi_query. The following example illustrates how + you use them together: + +

+CREATE TYPE foo_type AS (the_num INTEGER, the_text TEXT);
+
+CREATE OR REPLACE FUNCTION lotsa_md5 (INTEGER) RETURNS SETOF foo_type AS $$
+    use Digest::MD5 qw(md5_hex);
+    my $file = '/usr/share/dict/words';
+    my $t = localtime;
+    elog(NOTICE, "opening file $file at $t" );
+    open my $fh, '<', $file # ooh, it's a file access!
+        or elog(ERROR, "cannot open $file for reading: $!");
+    my @words = <$fh>;
+    close $fh;
+    $t = localtime;
+    elog(NOTICE, "closed file $file at $t");
+    chomp(@words);
+    my $row;
+    my $sth = spi_query("SELECT * FROM generate_series(1,$_[0]) AS b(a)");
+    while (defined ($row = spi_fetchrow($sth))) {
+        return_next({
+            the_num => $row->{a},
+            the_text => md5_hex($words[rand @words])
+        });
+    }
+    return;
+$$ LANGUAGE plperlu;
+
+SELECT * from lotsa_md5(500);
+

+ Normally, spi_fetchrow should be repeated until it + returns undef, indicating that there are no more + rows to read. The cursor returned by spi_query + is automatically freed when + spi_fetchrow returns undef. + If you do not wish to read all the rows, instead call + spi_cursor_close to free the cursor. + Failure to do so will result in memory leaks. +

+ spi_prepare(command, argument types) + + + spi_query_prepared(plan, arguments) + + + spi_exec_prepared(plan [, attributes], arguments) + + + spi_freeplan(plan) + +

+ spi_prepare, spi_query_prepared, spi_exec_prepared, + and spi_freeplan implement the same functionality but for prepared queries. + spi_prepare accepts a query string with numbered argument placeholders ($1, $2, etc) + and a string list of argument types: +

+$plan = spi_prepare('SELECT * FROM test WHERE id > $1 AND name = $2',
+                                                     'INTEGER', 'TEXT');
+

+ Once a query plan is prepared by a call to spi_prepare, the plan can be used instead + of the string query, either in spi_exec_prepared, where the result is the same as returned + by spi_exec_query, or in spi_query_prepared which returns a cursor + exactly as spi_query does, which can be later passed to spi_fetchrow. + The optional second parameter to spi_exec_prepared is a hash reference of attributes; + the only attribute currently supported is limit, which sets the maximum number of rows returned by a query. +

+ The advantage of prepared queries is that is it possible to use one prepared plan for more + than one query execution. After the plan is not needed anymore, it can be freed with + spi_freeplan: +

+CREATE OR REPLACE FUNCTION init() RETURNS VOID AS $$
+        $_SHARED{my_plan} = spi_prepare('SELECT (now() + $1)::date AS now',
+                                        'INTERVAL');
+$$ LANGUAGE plperl;
+
+CREATE OR REPLACE FUNCTION add_time( INTERVAL ) RETURNS TEXT AS $$
+        return spi_exec_prepared(
+                $_SHARED{my_plan},
+                $_[0]
+        )->{rows}->[0]->{now};
+$$ LANGUAGE plperl;
+
+CREATE OR REPLACE FUNCTION done() RETURNS VOID AS $$
+        spi_freeplan( $_SHARED{my_plan});
+        undef $_SHARED{my_plan};
+$$ LANGUAGE plperl;
+
+SELECT init();
+SELECT add_time('1 day'), add_time('2 days'), add_time('3 days');
+SELECT done();
+
+  add_time  |  add_time  |  add_time
+------------+------------+------------
+ 2005-12-10 | 2005-12-11 | 2005-12-12
+

+ Note that the parameter subscript in spi_prepare is defined via + $1, $2, $3, etc, so avoid declaring query strings in double quotes that might easily + lead to hard-to-catch bugs. +

+ Another example illustrates usage of an optional parameter in spi_exec_prepared: +

+CREATE TABLE hosts AS SELECT id, ('192.168.1.'||id)::inet AS address
+                      FROM generate_series(1,3) AS id;
+
+CREATE OR REPLACE FUNCTION init_hosts_query() RETURNS VOID AS $$
+        $_SHARED{plan} = spi_prepare('SELECT * FROM hosts
+                                      WHERE address << $1', 'inet');
+$$ LANGUAGE plperl;
+
+CREATE OR REPLACE FUNCTION query_hosts(inet) RETURNS SETOF hosts AS $$
+        return spi_exec_prepared(
+                $_SHARED{plan},
+                {limit => 2},
+                $_[0]
+        )->{rows};
+$$ LANGUAGE plperl;
+
+CREATE OR REPLACE FUNCTION release_hosts_query() RETURNS VOID AS $$
+        spi_freeplan($_SHARED{plan});
+        undef $_SHARED{plan};
+$$ LANGUAGE plperl;
+
+SELECT init_hosts_query();
+SELECT query_hosts('192.168.1.0/30');
+SELECT release_hosts_query();
+
+    query_hosts    
+-----------------
+ (1,192.168.1.1)
+ (2,192.168.1.2)
+(2 rows)
+

+ spi_commit() + + + spi_rollback() + +

+ Commit or roll back the current transaction. This can only be called + in a procedure or anonymous code block (DO command) + called from the top level. (Note that it is not possible to run the + SQL commands COMMIT or ROLLBACK + via spi_exec_query or similar. It has to be done + using these functions.) After a transaction is ended, a new + transaction is automatically started, so there is no separate function + for that. +

+ Here is an example: +

+CREATE PROCEDURE transaction_test1()
+LANGUAGE plperl
+AS $$
+foreach my $i (0..9) {
+    spi_exec_query("INSERT INTO test1 (a) VALUES ($i)");
+    if ($i % 2 == 0) {
+        spi_commit();
+    } else {
+        spi_rollback();
+    }
+}
+$$;
+
+CALL transaction_test1();
+

44.3.2. Utility Functions in PL/Perl

+ elog(level, msg) + +: + Emit a log or error message. Possible levels are + DEBUG, LOG, INFO, + NOTICE, WARNING, and ERROR. + ERROR + raises an error condition; if this is not trapped by the surrounding + Perl code, the error propagates out to the calling query, causing + the current transaction or subtransaction to be aborted. This + is effectively the same as the Perl die command. + The other levels only generate messages of different + priority levels. + Whether messages of a particular priority are reported to the client, + written to the server log, or both is controlled by the + log_min_messages and + client_min_messages configuration + variables. See Chapter 19 for more + information. +
+ quote_literal(string) + +: + Return the given string suitably quoted to be used as a string literal in an SQL + statement string. Embedded single-quotes and backslashes are properly doubled. + Note that quote_literal returns undef on undef input; if the argument + might be undef, quote_nullable is often more suitable. +
+ quote_nullable(string) + +: + Return the given string suitably quoted to be used as a string literal in an SQL + statement string; or, if the argument is undef, return the unquoted string "NULL". + Embedded single-quotes and backslashes are properly doubled. +
+ quote_ident(string) + +: + Return the given string suitably quoted to be used as an identifier in + an SQL statement string. Quotes are added only if necessary (i.e., if + the string contains non-identifier characters or would be case-folded). + Embedded quotes are properly doubled. +
+ decode_bytea(string) + +: + Return the unescaped binary data represented by the contents of the given string, + which should be bytea encoded. +
+ encode_bytea(string) + +: + Return the bytea encoded form of the binary data contents of the given string. +
+ encode_array_literal(array) + + + encode_array_literal(array, delimiter) +: + Returns the contents of the referenced array as a string in array literal format + (see Section 8.15.2). + Returns the argument value unaltered if it's not a reference to an array. + The delimiter used between elements of the array literal defaults to ", " + if a delimiter is not specified or is undef. +
+ encode_typed_literal(value, typename) + +: + Converts a Perl variable to the value of the data type passed as a + second argument and returns a string representation of this value. + Correctly handles nested arrays and values of composite types. +
+ encode_array_constructor(array) + +: + Returns the contents of the referenced array as a string in array constructor format + (see Section 4.2.12). + Individual values are quoted using quote_nullable. + Returns the argument value, quoted using quote_nullable, + if it's not a reference to an array. +
+ looks_like_number(string) + +: + Returns a true value if the content of the given string looks like a + number, according to Perl, returns false otherwise. + Returns undef if the argument is undef. Leading and trailing space is + ignored. Inf and Infinity are regarded as numbers. +
+ is_array_ref(argument) + +: + Returns a true value if the given argument may be treated as an + array reference, that is, if ref of the argument is ARRAY or + PostgreSQL::InServer::ARRAY. Returns false otherwise. +

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