15 April, 2022

A Fresh Look at AWR Reports --- 4 : SQL*Net Waits

 After the first three posts on CPU Usage, Redo Size and I/O Profile, I present my interpretation of SQL*Net waits.

From the same AWR Report as the first three reviews, I have extracted the specific SQL*Net waits from the Foreground Wait Events table:

SQL*Net Waits from the Foreground Waits Table

"SQL*Net more data from client" waits occur when the server process hasn't received the required data (e.g. an INSERT statement data) within 1 network packet and has to wait to receive more more.  "SQL*Net more data to client" is the reverse -- the client has requested, say a large row or dataset that doesn't fit into 1 network packet.  The network packet size is determined essentially by two parameters "Session Data Unit" (aka "SDU") and "Transport Data Unit" (aka "TDU").  When sending rows out of the server, it can automatically also do compression also. ARRAYSIZE (or Fetch Size, depending on your client code, in java) can also achieve some compression. TDU is obsolete in all recent versions but SDU can still be tuned.  For guidance on tuning SDU,  section 14.2 of the Tuning Guide.

"SQL*Net message to client" doesn't really show time taken to send the message (e.g. sql select statement result or execution of commit) to the client.  It is actually the time taken by the Server Process to put the results on to the Network Layer on the DB Server.  Beyond that, the Oracle Server Process does *not* how long it takes for the data to reach the client (whether a user desktop or an application server / middle-tier).  The actual time would depend on network latency and bandwidth.  So, you can interpret the number of occurrences of the Wait Event but NOT the time.

"SQL*Net break/reset" would be presented when an error occurs or an unhandled exception is raised. A high number of such waits indicate repeated occurrences of errors in SQL calls.

"SQL*Net message from client" may or may not be a problem (and that is why Oracle does not include it in % DB time).  There are different manifestations of this event :

1. The database server has sent a row or a set of rows (depending on arraysize or fetchsize) and is waiting for the client / application server to acknowledge and request the next set of rows.  The client could be a user who is running, say, SQL*Developer, and has seen the first 50 rows of a 1000 row result set but has walked away from his desk to get some coffee and has not yet requested the next set of 50 rows. (Yes, it is true that SQL*Developer and most clients do NOT fetch and present all the rows to the user immediately, this is is done in batches -- the parameter is configurable in most client or java programs).  The user may come back 5minutes or 30minutes later and ask for the next 50 rows.  Till then, the Oracle Server Process is "waiting" on this event.

2. It could also be an application server / middle tier that is formatting / sorting the rows or is writing the results to disk or is joining this set of rows with another data set (even from another source -- e.g. an ODBC connection, an Excel file etc) and is taking some milliseconds to do so.  Till then, the Oracle Server Process may be "waiting" on this event.

3. The database server has sent the results of a query (or success of execution of a DML) to the client / application server and is waiting for the next SQL call.  The next SQL call could be a COMMIT (or ROLLBACK) or any INSERT/UPDATE/DML statement.  It may come 1second later or it may come 30minutes later or it may come 10hours later.  As long as the client / application server has maintained a session on the Database, the Oracle Server Process will be "waiting" on this event.

Other than the above set of "Wait" Events relating to a client / application server, there may occur exactly the same Events relating to a Database Link. :  "SQL*Net more data from/to dblink",  "SQL*Net message from/to dblink" and SQL*Net break/reset to dblink".  They don't appear in my AWR because my example load does not include Database Links to / from other databases.  But you need to interpret them as well.  For example, if you have Database Links for periodic refresh's of Materialized Views between Databases, you may well see such "SQL*Net message" and "SQL*Net more data" waits relating to "dblink"s.

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