How to Debug Queries in Oracle

How do I debug slow Oracle SQL queries?

Identify and fix performance or logic issues in Oracle SQL using built-in tracing, explain plans, and session statistics.

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Description

Example H2

Example H3

Why does my Oracle query run slowly?

Slow queries usually suffer from missing indexes, bad join orders, or large data scans. Measure first with a reliable plan before rewriting.

How do I view a query’s execution plan?

Use EXPLAIN PLAN FOR followed by SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY). The plan shows join order, access paths, and estimated cost, guiding index or rewrite decisions.

Can I get real runtime statistics quickly?

In SQL*Plus or SQLcl, enable SET AUTOTRACE ON STATISTICS. The tool runs the query, returns rows, and prints consistent gets, physical reads, and elapsed time under 80 characters.

How do I trace exactly what the server does?

Enable SQL Trace at the session level: ALTER SESSION SET tracefile_identifier='orders_debug'; ALTER SESSION SET sql_trace=TRUE;. After running the query, disable tracing and analyze the generated .trc file with TKPROF.

When should I use DBMS_MONITOR?

Use DBMS_MONITOR.SESSION_TRACE_ENABLE for long-running jobs where you cannot alter SQL. It gives the same trace detail without changing application code.

What are best practices for debugging?

Always capture an execution plan, compare logical vs. physical reads, and test fixes on realistic data volumes. Keep changes isolated so you can revert quickly.

Why How to Debug Queries in Oracle is important

How to Debug Queries in Oracle Example Usage


--Identify why a customer’s large order query is slow
EXPLAIN PLAN FOR
SELECT c.name,
       SUM(oi.quantity * p.price) AS order_total
FROM   Customers   c
JOIN   Orders      o  ON o.customer_id = c.id
JOIN   OrderItems  oi ON oi.order_id    = o.id
JOIN   Products    p  ON p.id           = oi.product_id
WHERE  o.order_date BETWEEN DATE '2024-01-01' AND DATE '2024-03-31'
GROUP  BY c.name;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, NULL, 'ALL'));

How to Debug Queries in Oracle Syntax


--EXPLAIN PLAN
EXPLAIN PLAN SET STATEMENT_ID='debug_orders'
FOR
SELECT c.name, o.id, o.total_amount
FROM Customers c
JOIN Orders   o ON o.customer_id = c.id
WHERE o.order_date >= DATE '2024-01-01';
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE','debug_orders','BASIC +PREDICATE')); 

--AUTOTRACE (SQL*Plus)
SET AUTOTRACE ON EXPLAIN
SELECT *
FROM Orders
WHERE total_amount > 500;

--SQL TRACE (session)
ALTER SESSION SET tracefile_identifier='orders_trace';
ALTER SESSION SET sql_trace=TRUE;
--run query
ALTER SESSION SET sql_trace=FALSE;

--DBMS_MONITOR (package)
EXEC DBMS_MONITOR.SESSION_TRACE_ENABLE(waits=>TRUE, binds=>TRUE);
--run query
EXEC DBMS_MONITOR.SESSION_TRACE_DISABLE();

Common Mistakes

Relying on estimated plans only. Estimated costs ignore bind values and runtime statistics. Always compare EXPLAIN results with real AUTOTRACE or trace output.
Leaving SQL Trace on in production sessions. Continuous tracing creates huge files and IO overhead. Always disable tracing after you collect enough data.