问题
I am learning query optimization in Oracle and I know that trace file will create statistic about the query execution and EXPLAIN Plan of the query.
At the bottom of the trace file, it is EXPLAIN PLAN of the query. My first question is , does the part "time = 136437 us" show the time duration for the steps of query execution? what does "us" mean ? Is it unit of time?
In addition, can anyone explain what statistics such as count, cpu, elapsed , disk and query mean? I google and read Oracle doc about them already but I still can not understand it. Can anyone clarify the meaning of those stats more clearly?
Thanks in advance. I am new and sorry for my English.
回答1:
The smallest unit of data access in Oracle Database is a block. Not a row.
Each block can store many rows.
The database can access a block in current or consistent mode.
- Current = as the block exists "right now".
- Consistent = as the blocked existed at the time your query started.
The query and current columns report how many times the database accessed a block in consistent (query) and current mode.
When accessing a block it may already be in the buffer cache (memory). If so, no disk access is needed. If not, it has to do a physical read (pr). The disk column is a count of the total physical reads.
The stats for each line in the plan are the figures for that operation. Plus the sum of all its child operations.
In simple terms, the database processes the plan by accessing the first child first. Then passes the rows up to the parent. Then all the other child ops of that parent in order. Child operations are indented from their parent in the display.
So the database processed your query like so:
- Read 2,000 rows from CUSTOMER. This required 749 consistent block gets and 363 disk reads (cr and pr values on this row). This took 10,100 microseconds.
- Read 112,458 rows from BOOKING. This did 8,203 consistent reads and zero disk reads. This took 337,595 microseconds
- Joined these two tables together using a hash join. The CR, PR, PW (physical writes) and time values are the sum of the operations below this. Plus whatever work this operation did. So the hash join:
- did 8,952 - ( 749 + 8,203 ) = zero consistent reads
- did 363 - ( 363 + 0 ) = zero physical reads
- took 1,363,447 - ( 10,100 + 337,595 ) = 1,015,752 microseconds to execute
Notice that the CR & PR totals for the hash join match the query and disk totals in the fetch line?
The count column reports the number of times that operation happened. A fetch is a call to the database to get rows. So the client called the database 7,499 times. Each time it received ceil( 112,458 / 7,499 ) = 15 rows.
CPU is the total time in seconds the server's processors were executing that step. Elapsed is the total wall clock time. This is the CPU time + any extra work. Such as disk reads, network time, etc.
来源：https://stackoverflow.com/questions/52817309/how-to-understand-statistics-of-trace-file-in-oracle-such-as-cpu-elapsed-time