Unlock Oracle Cursor Insights: Master V$SQL, V$SQLAREA, and Tuning Parameters

1. Cursor Concept

From an Oracle DBA perspective, a cursor is any executable object stored in the library cache, including SQL statements and stored procedures. Developers typically consider only SQL statements as cursors. The DBA’s responsibility is to ensure these SQL statements execute efficiently, without concerning themselves with application logic.

2. Cursor‑Related Views

Typical three‑tier J2EE applications consist of a client, an application server, and a database server. When a Java program issues executeQuery("select * from Test"), the database server processes the SQL, records execution details in various dynamic performance views, and returns results to the client.

The most commonly referenced view is V$SQL , which provides a row per child cursor. Important columns include:

SQL_TEXT – the SQL statement text

SQL_ID – identifier of the statement

SHARABLE_MEM , PERSISTENT_MEM , RUNTIME_MEM – memory usage

SORTS – number of sorts performed

FETCHES , EXECUTIONS – fetch and execution counts

DISK_READS , BUFFER_GETS , CPU_TIME , ELAPSED_TIME – physical reads, logical reads, CPU time, and total elapsed time (key metrics for tuning)

When performance is unsatisfactory, DBAs should first examine statements with the highest DISK_READS , BUFFER_GETS , or CPU_TIME , or those with the greatest EXECUTIONS and PARSE_CALLS .

3. V$SQLAREA

V$SQLAREA aggregates data from all child cursors that share the same SQL text, effectively summarizing a parent cursor. Columns are similar to V$SQL, but values such as DISK_READS represent the sum of all child cursors. The view also includes VERSION_COUNT , indicating how many child cursors exist for the same parent – a high count may suggest missed sharing opportunities.

4. V$OPEN_CURSOR and the open_cursor Parameter

The view V$OPEN_CURSOR shows cursors currently opened in a session. When a cursor is parsed, a library‑cache lock is held until the cursor is explicitly closed. If many cursors remain open, the shared pool can become exhausted.

The initialization parameter open_cursor limits the number of simultaneously open cursors per session (default 50 in 9i, 300 in 10g). Exceeding this limit raises ORA‑01000: “maximum open cursors exceeded”. Cursors are automatically closed when the session ends.

5. CURSOR_SHARING Parameter

SQL statements that differ only by literal values (e.g., select * from table where id=1, ...id=2) generate separate cursors by default. Setting the cursor_sharing parameter can force sharing of execution plans:

EXACT – default; only identical text shares a cursor.

SIMILAR – if the plan for an existing cursor is optimal for the new statement, Oracle reuses the cursor.

FORCE – ignores plan optimality; any statement with the same text except for literal values shares a cursor.

The parameter can be changed at the session level with ALTER SESSION SET cursor_sharing = SIMILAR, affecting only that session.