Database Access from PL/Perl

PostgreSQL 8.3.6 Documentation
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# 40.2. Database Access from PL/Perl

Access to the database itself from your Perl function can be done via the function `spi_exec_query` described below, or via an experimental module `DBD::PgSPI` (also available at CPAN mirror sites). This module makes available a DBI-compliant database-handle named `$pg_dbh` that can be used to perform queries with normal DBI syntax.

PL/Perl provides additional Perl commands:

``spi_exec_query`(`query` [, `max-rows`])`
``spi_query`(`command`)`
``spi_fetchrow`(`cursor`)`
``spi_prepare`(`command`, `argument types`)`
``spi_exec_prepared`(`plan`)`
``spi_query_prepared`(`plan` [, `attributes`], `arguments`)`
``spi_cursor_close`(`cursor`)`
``spi_freeplan`(`plan`)`

  `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` 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);

  `spi_prepare`, `spi_query_prepared`, `spi_exec_prepared`, and `spi_freeplan` implement the same functionality but for prepared queries. 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 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 INTEGER 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 INTEGER 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.

  Normally, `spi_fetchrow` should be repeated until it returns `undef`, indicating that there are no more rows to read. The cursor 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.

``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 18 for more information.

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