[GDB] Debugging session PostgreSQL & AGE

Prerequisite

• Having a running instance of Postgresql server and (having it surely installed beside of AGE and debug flag is enabled)

The following commands are used for loading AGE

CREATE EXTENSION age;
LOAD ‘age’;
SET search_path = ag_catalog, "$user", public;

STEPS

• Open a psql session on a terminal

# create a test db if not having one
/usr/local/pgsql/bin/createdb test
# open psql
/usr/local/pgsql/bin/psql test

• Open a new tab for having gdb there

sudo gdb

• Get the backend process's pid

SELECT pg_backend_pid();

• assume output is 666
• Attach gdb to that process id

(gdb) attach 666

• Create break line at some where

(gdb) break src/backend/tcop/postgres.c:4483

• Execute a query at the backend session

select * from t where id > 1 and id < 10;

• Trace at the GDB (press n)

Quick guide to GDB

• To attach to a process: sudo gdb -p
• Attach breakpoint: break :
• Run until breakpoint: c
• Single step into the function: s
• Step over the function: n
• Print a value: p or p *
• Call a function: call
• Print postgres node: call pprint()
• Pressing enter key repeats last command.
• Include directory (source code): dir

References & Resources

• https://github.com/apache/age
• https://age.apache.org/
• https://blog.devgenius.io/stepping-through-postgresql-f40990685a56
• https://www.highgo.ca/2019/10/03/trace-query-processing-internals-with-debugger/