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Manual Reference Pages  -  DBD::PGLITE (3)

.ds Aq ’

NAME

DBD::PgLite - PostgreSQL emulation mode for SQLite

CONTENTS

SUMMARY



  use DBI;
  my $dbh = DBI->connect(dbi:PgLite:dbname=file);
  # The following PostgreSQL-flavoured SQL is invalid
  # in SQLite directly, but works using PgLite
  my $sql = q[
    SELECT
      news_id, title, cat_id, cat_name, sc_id sc_name,
      to_char(news_created,FMDD.FMMM.YYYY) AS ndate
    FROM
      news
      NATURAL JOIN x_news_cat
      NATURAL JOIN cat
      NATURAL JOIN subcat
    WHERE
      news_active = TRUE
      AND news_created > NOW() - INTERVAL 7 days
  ];
  my $res = $dbh->selectall_arrayref($sql,{Columns=>{}});
  # From v. 0.05 with full sequence function support
  my $get_nid = "SELECT NEXTVAL(news_news_id_seq)";
  my $news_id = $dbh->selectrow_array($get_nid);



DESCRIPTION

The module automatically and transparently transforms a broad range of SQL statements typical of PostgreSQL into a form suitable for use in SQLite. This involves both (a) parsing and filtering of the SQL; and (b) the addition of several PostgreSQL-compatible functions to SQLite.

Mainly because of datatype issues, support for many PostgreSQL features simply cannot be provided without elaborate planning and detailed metadata. Since this module is intended to be usable with any SQLite3 database, it follows that the emulation is limited in several respects. An overview of what works and what doesn’t is given in the following section on PostgreSQL Compatibility.

DBD::PgLite has support of a sort for stored procedures. This is described in the Extras section below. So are the few database functions defined by this module which are not in PostgreSQL. Finally, the Extras section contains a brief mention of the DBD::PgLite::MirrorPgToSQLite companion module.

If you do not want SQL filtering to be turned on by default for the entire session, you can connect setting the connection attribute FilterSQL to a false value:



  my $dbh = DBI->connect("dbi:PgLite:dbname=$fn",
                         undef, undef, {FilterSQL=>0});



To turn filtering off (or on) for a single statement, you can specify FilterSQL option as a statement attribute, e.g.:



  $dbh->do($sql, {FilterSQL=>0}, @bind);
  my $sth = $dbh->prepare($sql, {FilterSQL=>0});
  $res = $dbh->selectall_arrayref($sql, {FilterSQL=>0}, @bind);



It is possible to specify user-defined pre- and postfiltering routines, both globally (by specifying them as attributes of the database handle) and locally (by specifying them as statement attributes):



  $dbh = DBI->connect("dbi:PgLite:$file",undef,undef,
                      {prefilter=>\&prefilter});
  $res = $dbh->selectall_arrayref($sql,
                                  {postfilter=>\&postfilter},
                                  @bind_values);



The pre-/postfiltering subroutine receives the SQL as parameter and is expected to return the changed SQL.

STATUS OF THE MODULE

This module was initially developed using SQLite 3.0 and PostgreSQL 7.3, but it should be fully compatible with newer versions of both SQLite (3.1 and 3.2 have been tested) and PostgreSQL (8.1 has been tested).

Support for SELECT statements and the WHERE-conditions of DELETE and UPDATE statements is rather good, though still incomplete. The module especially focuses on NATURAL JOIN differences and commonly used, built-in PostgreSQL functions.

Support for inserted/updated values in INSERT and UPDATE statements could use some improvement but is useable for simple things.

There is no support for differences in DDL.

The SQL transformations used are not based on a formal grammar but on applying simple regular expressions. An obvious consequence of this is that they may depend excessively on the author’s SQL style. YMMV. (I would however like you to contact me if you come across some SQL statements which you feel should work but that don’t).

The development of this module has been driven by personal needs, and so is likely to be even more one-sided than the above description suggests.

POSTGRESQL COMPATIBILITY

In this section, the PostgreSQL functions and operators supported by the module are enumerated.

    Regex operators

o The regex operators ~, ~*, !~ and !~* are transformed into calls to the user-defined function matches(). The regex flavour supported is Perl, not plain vanilla POSIX, so some incompatibilities may arise.
o Note that for ease of parsing, whitespace before and after the operator is required for the filtering to succeed. So col ~ ’pat’ works, but col~’pat’ doesn’t.
o SIMILAR TO is not supported.
o ILIKE is quietly changed to LIKE. LIKE in SQLite is case-insensitive for 7-bit characters. In future, ILIKE will probably be handled more elegantly, and LIKE will be redefined so as to be more like PostgreSQL.

    Math Functions

o Added: abs, cbrt, ceil, degrees, exp, floor, ln, log (1- and 2-argument forms), mod, pi, pow, radians, sign, sqrt, trunc (1- and 2-argument forms), acos, asin, atan, atan2, cos, cot, sin, tan.
o random() exists in SQLite but was redefined to conform better with PostgreSQL in terms of value range. setseed() was also added, but is not entirely compatible with PostgreSQL in the sense that setting the random seed does not engender the same sequence of pseudo-random numbers as it would in PostgreSQL.
o SQLite already has a handful of mathematical functions which have been left alone, notably round() (1- and 2-argument forms).

    String Functions

The only string functions which are present natively in SQLite are substr(), lower() and upper(). These have been left alone. Added functions are the following:
o ascii, bit_length, btrim, char_length, character_length, chr, convert (1- and 2-arg), decode, encode, initcap, length, lpad, ltrim, md5, octet_length, position, pg_client_encoding (always ’SQL_ASCII’), quote_ident, quote_literal, repeat, replace, rpad, rtrim, split_part, strpos, substring(string,offset,length), substring(string from pattern), to_ascii (assumes latin-1 input), to_hex, translate, trim.
Except for convert(), where another input encoding can be specified explicitly, these functions all assume that the strings are in an 8-bit character set, preferably iso-8859-1.

The little-used idiom substring(string from pattern for escape) (where ’pattern’ is not a POSIX regular expression but a SQL pattern) is not supported. Otherwise support for string functions is pretty complete.

    Data Type Formatting Functions

The implementation of these functions is impeded by the sparse type system employed by SQLite. Workarounds are possible, however, so this area will probably be better covered in future.
o to_char(timestamp, format) is mostly supported. There is support for most formatting strings (all except ’Y,YYY’, ’IYYY’, ’IYY’, ’IY’, ’I’, ’J’, ’TZ’, and ’tz’). The FM prefix is supported for ’MM’, ’DD’ and ’HH*’, but not otherwise. Other prefixes are not supported.
o to_char(interval, format) and to_char(number, format) are not currently supported. Nor are to_date(), to_timestamp() and to_number() (yet).

    Date/Time Functions

Again, SQLite’s intrinsically bad support for dates and intervals makes this area somewhat hard to cover properly. Function support is as follows; also note the caveats below:
o Supported: now, current_date, current_time, current_datetime, date_part (with timestamps, not intervals), date_trunc, extract (with timestamps, not intervals), localtime, localtimestamp, timeofday.
o Not supported: age, isfinite, overlaps.
Versions of SQLite 3.1 and later support some of these functions, e.g. current_date. In these versions the built-in will be overridden.

The module makes no distinction between time/timestamp with and without time zone. It is assumed that times and timestamps are either all GMT or all localtime; time zone information is silently discarded. This may change later.

Support for calculations with dates and intervals is still very limited. Basically, what is supported are expressions of the form expr +/- interval ’descr’ where expr reduces to a timestamp or date value.

If a transaction is started with begin_work(), the time as represented by now() and friends is frozen in the same way as in PostgreSQL until commit() or rollback() are called. A transaction started by simply running the SQL statement BEGIN does not, however, trigger this behaviour. Nor is the time automatically unfrozen when an error occurs during a transaction; you need to catch exceptions and call rollback() manually.

    Sequence Manipulation Functions

o There is now full support for all explicit invocations of the sequence functions nextval(), setval(), currval() and lastval(). Sequences are emulated using the table pglite_seq. (This works even with multiple connections to the same database file, some of which are using transactions, since SQLite transactions lock the whole database file, luckily eliminating any risk of two connections getting the same value from a nextval() call).

Please be aware that sequences are autogenerated if they do not exist. Be careful to specify the appropriate sequence names or you will get unexpected results.

If a sequence being autogenerated ends with ’_seq’ and has a name which seems to match an existing table + an integer column from that table (tablename_colname_seq), it is given an initial value based on the maximum value in the column in question.

There is as yet no support for CREATE SEQUENCE statements. Use the autogeneration feature to create sequences.

Implicit calls to NEXTVAL() by omitting the serial column from the column list in an INSERT are caught in most cases. The main conditions that must be fulfilled for this to work are: (1) that the column in question is an integer column which is the sole primary key on the table; and (2) that the statement is a normal INSERT with a column list and a VALUES clause (and not, e.g., a statement of the form INSERT INTO x SELECT * FROM y).

There is as yet no interaction with the SQLite builtin autoincrement/last_insert_rowid() functionality in connection with the sequence function support.

    Aggregate Functions

o max(), min(), count() and sum() are already supported by SQLite and have been left alone. Note that the construct count(distinct colname) is not supported unless the SQLite version being used supports it (3.2.6 and later).
o avg() has been added.
o stddev() and variance() are not supported.

    A Note on Casting

Casting using the construct ::datatype is not supported in general. However, ::int, ::date and ::bool should work as expected. All other casts are silently discarded.

    A Note on Booleans

This module assumes that booleans will be stored as numeric values in the SQLite database. SQLite interprets 0 as false and any non-zero numeric value as true. Accordingly, expressions such as = TRUE and = ’t’ are simply removed in SELECT and DELETE statements. Likewise, expr = FALSE is turned into NOT expr before being passed on to SQLite.

In INSERT and DELETE statements, TRUE and FALSE (as well as ’t’::bool and ’f’::bool - but not ’t’ and ’f’ by themselves) are turned into 1 and 0.

    Current_user etc.

The functions current_user(), session_user() and user() - with or without parentheses - all mean the same thing. They return the username of the effective uid.

    Other Functions

The main groups of other functions (not supported by this module at all) are:
o Database/user information functions: Aside from current_user/session_user/user, which were mentioned above, no functions in this group are supported. This includes current_database(), current_schema(), all functions with names starting with ’pg_’ and ’has_’, obj_description and col_description. See http://www.postgresql.org/docs/current/static/functions-misc.html
o Array functions are not implemented - see http://www.postgresql.org/docs/current/static/functions-array.html
o Binary string (BYTEA) functions are not implemented - see http://www.postgresql.org/docs/current/static/functions-binarystring.html
o Geometric functions are not implemented - see http://www.postgresql.org/docs/current/static/functions-geometry.html
o Network Address Functions are not implemented - see http://www.postgresql.org/docs/current/static/functions-net.html

EXTRAS

    Stored Procedures

If the active database file contains a table called pglite_functions, the module assumes that it will have the following structure:



  CREATE TABLE pglite_functions (
    name   TEXT,   -- name  of the function
    argnum INT,    -- number of arguments (-1 means any number)
    type   TEXT,   -- can be sql or perl
    sql    TEXT,   -- the body of the function
    PRIMARY KEY (name, argnum)
  );



In the case of a SQL-type function, it can contain syntax supported through the module (and not directly by SQLite). The numeric arguments ($1-$9) customary in PostgreSQL are supported, so that in many cases simple functions will be directly transferrable from pg_proc in a PostgreSQL database.

An instance of a SQL snippet which would work as a function body both in PostgreSQL and PgLite (e.g. with the function name ’full_price_descr’):



  SELECT TRIM(group_name||: ||price_description)
    FROM price_group NATURAL JOIN price
    WHERE price_id = $1



As for perl-type functions, the function body is simply the text of a subroutine. Here is a simple example of a function body for the function ’commify’, which takes two arguments: the number to be formatted and the desired number of decimal places:



  sub {
    my ($num,$dp) = @_;
    my $format = "%.${dp}f";
    $num = scalar reverse(sprintf $format, $num);
    my $rest = $1 if $num =~ s/^(\d+)\.//;
    $num =~ s/(...)/$1,/g;
    $num = "$rest.$num" if $rest;
    return scalar reverse($num);
  }



    Non-Pg Functions

matches(), imatches(): These functions are used behind the scenes to implement support for the ’~’ regex-matching operator and its variants. They take two arguments, a string and a regular expression. matches() is case sensitive, imatches() isn’t.
matches_safe(), imatches_safe(): These work in the same way as matches() and imatches() except that metacharacters are escaped in the regex argument. They are therefore in many cases more suitable for user input and other untrusted sources.
lower_latin1(): Depending on platform, lower() and upper() may not transform the case of non-ascii characters despite a proper locale being defined in the environment. This functions assumes that a Latin-1 locale is active and returns a lower-case version of the input given this assumption.
localeorder(): DBD::SQLite does not provide access to defining SQLite collation functions. This is a workaround for a specific case where this limitation can be an issue. Given a Latin-1 encoded string, it returns a string of hex digits which can be ascii-sorted in the ordinary way. The resulting row order will be in accordance with the currently active locele - but only if the locale is Latin-1 based. The sort is case-insensitive.
locale(): An information function simply returning the name of the current locale. The module sets the locale based on the environment variables $ENV{LC_COLLATE}, $ENV{LC_ALL}, $ENV{LANG}, and $ENV{LC_CTYPE}, in that order. Currently it is not possible to use different locales for character type and collation, as far as the module is concerned.

    DBD::PgLite::MirrorPgToSQLite

The companion module, DBD::PgLite::MirrorPgToSQLite, may be of use in conjunction with this module. It can be used for easily mirroring specific tables from a PostgreSQL database, moving views and (some) functions as well if desired.

CAVEATS

Some functions defined by the module are not suitable for use with UTF-8 data and/or in an UTF-8 locale. (This, however, would be rather easy to change if you’re willing to sacrifice proper support for 8-bit locales such as iso-8859-1).

Please do not make the mistake of using this module for an important production system - too much can go wrong. But as a development tool it can be useful, and as a toy it can be fun...

TODO

There is a lot left undone. The next step is probably to handle non-SELECT statements better.

SEE ALSO

DBI, DBD::SQLite, DBD::Pg, DBD::PgLite::MirrorPgToSQLite;

THANKS TO

Johan Vromans, for encouraging me to improve the sequence support.

AUTHOR

Baldur Kristinsson (bk@mbl.is), 2006.



 Copyright (c) 2006 Baldur Kristinsson. All rights reserved.
 This program is free software; you can redistribute it and/or
 modify it under the same terms as Perl itself.



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