Weird issue with truncation of values in array with some tables

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Weird issue with truncation of values in array with some tables

Mike Martin
Hi
I have come across a weird issue with truncation of text in an array (in this case using pg_indexes view)

This query truncates the second array element at 63 characters
SELECT ARRAY[indexname,indexdef] FROM pg_indexes

However reversing the order doesn't truncate

SELECT ARRAY[indexdef,indexname] FROM pg_indexes

Anyone know why this behaviour occurs?


thanks

Mike
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Re: Weird issue with truncation of values in array with some tables

David G Johnston
On Sun, Aug 16, 2020, 06:49 Mike Martin <[hidden email]> wrote:
Hi
I have come across a weird issue with truncation of text in an array (in this case using pg_indexes view)

This query truncates the second array element at 63 characters
SELECT ARRAY[indexname,indexdef] FROM pg_indexes

However reversing the order doesn't truncate

SELECT ARRAY[indexdef,indexname] FROM pg_indexes

Anyone know why this behaviour occurs?

It's likely related to the following:


The system uses no more than NAMEDATALEN-1 bytes of an identifier; longer names can be written in commands, but they will be truncated. By default, NAMEDATALEN is 64 so the maximum identifier length is 63 bytes.

David J.

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Re: Weird issue with truncation of values in array with some tables

Tom Lane-2
In reply to this post by Mike Martin
Mike Martin <[hidden email]> writes:
> I have come across a weird issue with truncation of text in an array (in
> this case using pg_indexes view)
> This query truncates the second array element at 63 characters
> SELECT ARRAY[indexname,indexdef] FROM pg_indexes
> However reversing the order doesn't truncate
> SELECT ARRAY[indexdef,indexname] FROM pg_indexes
> Anyone know why this behaviour occurs?

indexname is of type name, indexdef is of type text, and the rules
for inferring the type of an array[] construct are such that the
first element's type wins in these cases.

regression=# SELECT pg_typeof(ARRAY[indexname,indexdef]) FROM pg_indexes limit 1;
 pg_typeof
-----------
 name[]
(1 row)

regression=# SELECT pg_typeof(ARRAY[indexdef,indexname]) FROM pg_indexes limit 1;
 pg_typeof
-----------
 text[]
(1 row)

You could insert an explicit cast to text to avoid the truncation
of indexdef to name:

regression=# SELECT pg_typeof(ARRAY[indexname::text,indexdef]) FROM pg_indexes limit 1;
 pg_typeof
-----------
 text[]
(1 row)

The documentation about that is here:
https://www.postgresql.org/docs/current/typeconv-union-case.html
although looking at this example it seems like that description isn't
telling the full truth.

                        regards, tom lane


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Re: Weird issue with truncation of values in array with some tables

Mike Martin-2
Thanks! 
Possibly the use of name type in pg_tables could be emphasized and maybe a note about name type in docs somewhere 

On Sun, 16 Aug 2020, 18:12 Tom Lane, <[hidden email]> wrote:
Mike Martin <[hidden email]> writes:
> I have come across a weird issue with truncation of text in an array (in
> this case using pg_indexes view)
> This query truncates the second array element at 63 characters
> SELECT ARRAY[indexname,indexdef] FROM pg_indexes
> However reversing the order doesn't truncate
> SELECT ARRAY[indexdef,indexname] FROM pg_indexes
> Anyone know why this behaviour occurs?

indexname is of type name, indexdef is of type text, and the rules
for inferring the type of an array[] construct are such that the
first element's type wins in these cases.

regression=# SELECT pg_typeof(ARRAY[indexname,indexdef]) FROM pg_indexes limit 1;
 pg_typeof
-----------
 name[]
(1 row)

regression=# SELECT pg_typeof(ARRAY[indexdef,indexname]) FROM pg_indexes limit 1;
 pg_typeof
-----------
 text[]
(1 row)

You could insert an explicit cast to text to avoid the truncation
of indexdef to name:

regression=# SELECT pg_typeof(ARRAY[indexname::text,indexdef]) FROM pg_indexes limit 1;
 pg_typeof
-----------
 text[]
(1 row)

The documentation about that is here:
https://www.postgresql.org/docs/current/typeconv-union-case.html
although looking at this example it seems like that description isn't
telling the full truth.

                        regards, tom lane