How to hash a large amount of data within Postgres? - Mailing list pgsql-general

Hi all,

I'm one of those people still storing user uploaded files within
Postgres instead of some file system and over the years this increased
to a point where individual uploads of multiple GiB are reached.

Some years ago I implemented some SQL to read all files, build a table
of SHA256 hashes and tell me how much data is redundant. The goal was
to have a look at which files share the same hash with different LOIDs
and optionally change that, so that all those files are only stored
once on the end.

While the approach was pretty naive, because it simply read all files
into memory to calculate the hashes, I'm somewhat sure it worked in
the past with Postgres 9.6. The executing server had enough free RAM
available as well to process the at most ~4 GiB large files one after
another.

I tried that SQL today with Postgres 11 on UB 18.04 and it failed:

> [Code: 0, SQL State: XX000]  FEHLER: invalid memory alloc request size 1898107949
>   Wobei: PL/pgSQL-Funktion loid_hash_calc(oid,text)[...]

> PostgreSQL
> 11.12 (Ubuntu 11.12-1.pgdg18.04+1)
> PostgreSQL JDBC Driver
> 42.2.9

I searched regaridng that issue and only found two relevant results:
Corrupted rows for some reason and simply size restrictions when
allocating memory. The latter is more likely than the former in my
case, as the restrictions seems to be 1 GiB and I do have larger
files.

I'm doing the following simply currently, because I didn't find any
interfaces allowing to forward blocks of data, LOIDs, file descriptors
or anything like that working with smaller buffers or alike.

> fd      := lo_open( loid,  INV_READ);
> size    := lo_lseek(fd, 0, SEEK_END);
> PERFORM    lo_lseek(fd, 0, SEEK_SET);

> hashBin := digest(loread(fd, size), algorithm);
> hashHex := encode(hashBin,          'hex');

So, is there any way to work around the problem I have currently? Can
I increase the memory restriction somewhere in the config? Are there
any functions available working with blocks of data I'm missing now? I
didn't find any state maintainig HASH-calls.

Thanks!

Mit freundlichen Grüßen

Thorsten Schöning

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Für Rückfragen stehe ich Ihnen sehr gerne zur Verfügung.

Mit freundlichen Grüßen

Thorsten Schöning


Tel: 05151 9468 0
Fax: 05151 9468 88
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