Oracle wait events are statistics that a server process or thread increments when it waits for an operation to complete in order to continue its processing.
Here are some of the most common Oracle wait events explained.
Table of Contents
Buffer busy waits
This event occurs when a session wants to access a database block in the buffer cache but it cannot because the buffer is busy.
In simple terms: Imagine you have one special video game console that only one person can use at a time.
If you want to play and someone else is already using it, you have to wait until they finish.
In Oracle, the “buffer” is like that video game console—it holds important data in memory.
When a task wants to use a piece of that data (a database block) but it’s already busy being used by someone else, the task has to wait.
This waiting is called a “buffer busy wait.”
Free buffer waits
This event occurs mainly when a server process is trying to read a new buffer into the buffer cache but too many buffers are either pinned or dirty and thus unavailable for reuse.
The session posts to DBWR then waits for DBWR to create free buffers by writing out dirty buffers to disk.
In simple terms: Imagine you have a toy shelf that’s completely full.
Now, you want to put a new toy on it, but there’s no room because all the spots are taken by toys that are either being played with or need to be cleaned up first.
Db file sequential read
This event shows a wait for a foreground process while doing a sequential read from the database.
The I/O is generally issued as a single I/O request to the OS; the wait blocks until the I/O request completes.
In simple terms: Imagine you have a favorite picture book, but it’s kept on a high shelf. When you want to look at a specific page, you ask a friend to grab it for you. While your friend fetches the page, you sit and wait until they come back with it.
Db file scattered read
This is the same type of event as “db file sequential read”, except that Oracle will read multiple data blocks.
Multi-block reads are typically used on full table scans.
The name “scattered read” refers to the fact that multiple blocks are read into database block buffers that are ‘scattered’ throughout memory.
In simple terms: Imagine you have a huge picture book and you want to look through many pages quickly.
Instead of asking for one page at a time (like in a sequential read), you ask for a whole bunch of pages at once.
These pages might come from different parts of the book, so when they arrive, they’re spread out or “scattered” all over your reading table.
Enqueue waits
This metric represents the total number of waits per second that occurred during an enqueue convert or get because the enqueue get was deferred.
In simple terms: Imagine you have one special slide at the playground that everyone wants to use.
When you get to the slide and find someone else is already on it, you have to wait your turn.
Each time you wait because the slide is busy, that’s like an “enqueue wait.”
Log buffer space
Wait for space in the log buffer because we are writing into the log buffer faster than lgwr can write it out.
In simple terms: Imagine you have a special notepad where you quickly jot down notes.
There’s a helper, LGWR, whose job is to take your notes and write them into a big, safe book.
If you write notes too fast and the notepad fills up before your helper can clear some space, you have to pause and wait until there’s room again.
This waiting time is called a “log buffer space” wait.
Log file sync
It is the wait for LGWR to write to the redo log files.
In simple terms: Imagine you’re playing a game where every move you make needs to be written down in a special diary to remember it.
There’s a friend called LGWR whose job is to write down each move.
If you make a move and then have to wait because your friend is still writing the previous move in the diary, that’s a “log file sync” wait.
It means you’re pausing until LGWR finishes writing your move down in the log files.
I hope this was helpful.