I'm working on reading large BYTEA fields from PostgreSQL 8.1. (For
legacy reasons, it's unattractive to move them to large objects.) I'm
using JDBC, and as various people have pointed out
<http://archives.postgresql.org/pgsql-jdbc/2005-06/msg00138.php>, the
standard stream-style access method runs out of memory for large BYTEAs.
Karsten Hilbert mentions using SUBSTRING to read these BYTEA fields a
chunk at a time
<http://archives.postgresql.org/pgsql-general/2005-01/msg00032.php>.
I've tried this, and indeed it works. (Once I corrected for the 1-based
indexing ;-))
My question is about performance in the postgres server. When I execute
"SELECT SUBSTRING (my_bytea FROM ? FOR ?) FROM my_table WHERE id = ?",
does it fetch the whole BYTEA into memory? Or does it access only the
pages that contain the requested substring?
Vance
