Prepared statement leak

We seem to have identified a prepared statement leak in the JDBC driver.  The
actual application runs through Hibernate, so we have attempted to isolate
the problem here.  In the field, the problem causes the PostgreSQL server to
run out of memory in linear time.

We have tested with versions

postgresql-8.0-315.jdbc3.jar
postgresql-8.1-405.jdbc3.jar
postgresql-8.2dev-501.jdbc3.jar

with no difference.

The two prerequisites for this problem to appear are:

1. PreparedStatement objects are rebound with different types, which causes
the JDBC driver to replan the statement.

2. Batches are used.

Attached is a test program that exhibits this.  Create a database test with a
table

create table test (a int, b text);

and run the program.

I was initially at a loss about how to debug this problem so I wrote a patch
for the backend to trace the prepared statements table, which turns out to
clearly show the leak.  The attached patch is for PostgreSQL 8.0 and writes
the interesting output if log_min_messages is debug1.  (There is some extra
memory debugging code in there that I would not advise you to use.)

Running the test program you see this in the server log near the end:

DEBUG:  prepared statement hash table size = 100

It should be near 0, of course.

In the JDBC driver log output you see blocks like this:

batch execute 3 queries,
handler=org.postgresql.jdbc2.AbstractJdbc2Statement$BatchResultHandler@750159,
maxRows=0, fetchSiz
 FE=> Parse(stmt=S_1,query="INSERT INTO test VALUES ($1, $2)",oids={23,1042})
 FE=> Bind(stmt=S_1,portal=null,$1=<101>,$2=<test>)
 FE=> Describe(portal=null)
 FE=> Execute(portal=null,limit=1)
 FE=> Parse(stmt=S_2,query="INSERT INTO test VALUES ($1, $2)",oids={23,1043})
 FE=> Bind(stmt=S_2,portal=null,$1=<201>,$2=<test>)
 FE=> Describe(portal=null)
 FE=> Execute(portal=null,limit=1)
 FE=> Parse(stmt=S_3,query="INSERT INTO test VALUES ($1, $2)",oids={23,1042})
 FE=> Bind(stmt=S_3,portal=null,$1=<301>,$2=<test>)
 FE=> Describe(portal=null)
 FE=> Execute(portal=null,limit=1)
 FE=> Sync
 <=BE ParseComplete [S_3]
 <=BE BindComplete [null]
 <=BE NoData
 <=BE CommandStatus(INSERT 20239 1)
 <=BE ParseComplete [S_3]
 <=BE BindComplete [null]
 <=BE NoData
 <=BE CommandStatus(INSERT 20240 1)
 <=BE ParseComplete [S_3]
 <=BE BindComplete [null]
 <=BE NoData
 <=BE CommandStatus(INSERT 20241 1)
 <=BE ReadyForQuery(I)

Notice that it seems to forget out S_1 and S_2 along the way.

--
Peter Eisentraut
http://developer.postgresql.org/~petere/