BUG #8441: Recursive function in plpgsql does not seem to handle null values correctly

Started by Nonameover 12 years ago4 messagesbugs

tvees@davincigroep.nl

over 12 years ago

The following bug has been logged on the website:

Bug reference: 8441
Logged by: Tom van Ees
Email address: tvees@davincigroep.nl
PostgreSQL version: 9.0.4
Operating system: Windows Server 2008 R2
Description:

The Levenshtein function can only handle strings with length 255 or less.
I needed a Levenshtein function that could handle longer strings.
Therefore I wrote the following udf:

CREATE OR REPLACE FUNCTION longlevenshtein (string1 character varying
(1000000), string2 character varying (1000000)) RETURNS integer AS $$
BEGIN
IF (length(coalesce($1, '')) = 0 AND length(coalesce($2, '')) = 0) THEN
RETURN 0;
ELSEIF ($1 IS NULL and $2 IS NOT NULL and length($2) > 0) THEN
RETURN length($2);
ELSEIF ($2 IS NULL and $1 IS NOT NULL and length($1)> 0) THEN
RETURN length($1);
ELSEIF length($1) = 0 AND length(coalesce($2, '')) > 0 THEN
RETURN length(coalesce($2, ''));
ELSEIF length($1) > 0 AND (length($2) = 0 or $2 is null) THEN
RETURN length(coalesce($1, ''));
ELSE
RETURN (Levenshtein(SUBSTRING($1 FROM 1 FOR 254), SUBSTRING($2 FROM 1
for 254)) + longlevenshtein(coalesce(SUBSTRING($1 FROM 255), ''),
coalesce(SUBSTRING($2 FROM 255), '')));
END IF;
END;
$$ LANGUAGE plpgsql;

When I invoke this function with
SELECT longlevenshtein(null, 'foobar')
I get a ERROR: stack depth limit exceeded
while I expected the return value 6

--
Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-bugs

Pavel Stehule

pavel.stehule@gmail.com

over 12 years ago

In reply to: Noname (#1)

Re: BUG #8441: Recursive function in plpgsql does not seem to handle null values correctly

Hello

it works on 9.1.9

postgres=# SELECT longlevenshtein(null, 'foobar');
longlevenshtein
-----------------
6

Regards

Pavel

P.S. unlimitted varchar is "text" type in Postgres

2013/9/9 <tvees@davincigroep.nl>

Show quoted text

The following bug has been logged on the website:

Bug reference: 8441
Logged by: Tom van Ees
Email address: tvees@davincigroep.nl
PostgreSQL version: 9.0.4
Operating system: Windows Server 2008 R2
Description:

The Levenshtein function can only handle strings with length 255 or less.
I needed a Levenshtein function that could handle longer strings.
Therefore I wrote the following udf:

CREATE OR REPLACE FUNCTION longlevenshtein (string1 character varying
(1000000), string2 character varying (1000000)) RETURNS integer AS $$
BEGIN
IF (length(coalesce($1, '')) = 0 AND length(coalesce($2, '')) =
0) THEN
RETURN 0;
ELSEIF ($1 IS NULL and $2 IS NOT NULL and length($2) > 0) THEN
RETURN length($2);
ELSEIF ($2 IS NULL and $1 IS NOT NULL and length($1)> 0) THEN
RETURN length($1);
ELSEIF length($1) = 0 AND length(coalesce($2, '')) > 0 THEN
RETURN length(coalesce($2, ''));
ELSEIF length($1) > 0 AND (length($2) = 0 or $2 is null) THEN
RETURN length(coalesce($1, ''));
ELSE
RETURN (Levenshtein(SUBSTRING($1 FROM 1 FOR 254), SUBSTRING($2
FROM 1
for 254)) + longlevenshtein(coalesce(SUBSTRING($1 FROM 255), ''),
coalesce(SUBSTRING($2 FROM 255), '')));
END IF;
END;
$$ LANGUAGE plpgsql;

When I invoke this function with
SELECT longlevenshtein(null, 'foobar')
I get a ERROR: stack depth limit exceeded
while I expected the return value 6

--
Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-bugs

Andres Freund

andres@anarazel.de

over 12 years ago

In reply to: Pavel Stehule (#2)

Re: BUG #8441: Recursive function in plpgsql does not seem to handle null values correctly

Hi,

On 2013-09-09 17:26:48 +0200, Pavel Stehule wrote:

it works on 9.1.9

That's probably a question of the configured/detected max_stack_depth.

I have quite some doubts such a naive implementation implemented in
plgsql will work for strings > 255 chars in sensible manner though.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

--
Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-bugs

Tom Lane

tgl@sss.pgh.pa.us

over 12 years ago

In reply to: Noname (#1)

Re: BUG #8441: Recursive function in plpgsql does not seem to handle null values correctly

tvees@davincigroep.nl writes:

CREATE OR REPLACE FUNCTION longlevenshtein (string1 character varying
(1000000), string2 character varying (1000000)) RETURNS integer AS $$
BEGIN
IF (length(coalesce($1, '')) = 0 AND length(coalesce($2, '')) = 0) THEN
RETURN 0;
ELSEIF ($1 IS NULL and $2 IS NOT NULL and length($2) > 0) THEN
RETURN length($2);
ELSEIF ($2 IS NULL and $1 IS NOT NULL and length($1)> 0) THEN
RETURN length($1);
ELSEIF length($1) = 0 AND length(coalesce($2, '')) > 0 THEN
RETURN length(coalesce($2, ''));
ELSEIF length($1) > 0 AND (length($2) = 0 or $2 is null) THEN
RETURN length(coalesce($1, ''));
ELSE
RETURN (Levenshtein(SUBSTRING($1 FROM 1 FOR 254), SUBSTRING($2 FROM 1
for 254)) + longlevenshtein(coalesce(SUBSTRING($1 FROM 255), ''),
coalesce(SUBSTRING($2 FROM 255), '')));
END IF;
END;
$$ LANGUAGE plpgsql;

When I invoke this function with
SELECT longlevenshtein(null, 'foobar')
I get a ERROR: stack depth limit exceeded

Worksforme. You sure you transcribed the function accurately?

Note however that sufficiently long input strings *will* drive this
function to stack overrun, if you don't run out of heap memory first
(I think the heap consumption will be O(N^2) ...). Consider rewriting
it with a loop rather than recursion.

regards, tom lane

--
Sent via pgsql-bugs mailing list (pgsql-bugs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-bugs