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Oracle Inline views

Oracle Tips by Burleson Consulting
Don Burleson

Question:  What is an Oracle "in-line view?  How do I use an inline view?

Answer:  The inline view is a construct in Oracle SQL where you can place a query in the SQL FROM, clause, just as if the query was a table name.

Oracle has long supported the notion of an 'in-line view,' whereby a subquery can be placed in the FROM clause, just as if it were a table name. There's an Oracle query displaying tablespace sizes:

A common use for in-line views in Oracle SQL is to simplify complex queries by removing join operations and condensing several separate queries into a single query.

The best example of the in-line view is the common Oracle DBA script that is used to show the amount of free space and used space within all Oracle tablespaces. Let's take a close look at this SQL to see how it works. Carefully note that the FROM clause in this SQL query specifies two sub-queries that perform summations and grouping from two views, dba_data_files, and dba_free_space.

In ANSI standard SQL, it is quite difficult to compare two result sets that are summed together in a single query, and this is a common problem with Oracle SQL where specific values must be compared to a summary.  Without the use of an in-line view, several separate SQL queries would need to be written, one to compute the sums from each view and another to compare the intermediate result sets.

This is a great report for display the actual amount of free space within an Oracle tablespace.

column "Tablespace" format a13
column "Used MB"    format 99,999,999
column "Free MB"    format 99,999,999
colimn "Total MB"   format 99,999,999

   fs.tablespace_name                          "Tablespace",
   (df.totalspace - fs.freespace)              "Used MB",
   fs.freespace                                "Free MB",
   df.totalspace                               "Total MB",
   round(100 * (fs.freespace / df.totalspace)) "Pct. Free"
      round(sum(bytes) / 1048576) TotalSpace
   group by
   ) df,
      round(sum(bytes) / 1048576) FreeSpace
   group by
   ) fs
   df.tablespace_name = fs.tablespace_name;

This SQL quickly compares the sum of the total space within each tablespace to the sum of the free space within each tablespace. Here is a sample of the output:

Basically, this query needs to compare the sum of total space within each tablespace with the sum of the free space within each tablespace.

Here is the output from this in-line view query against the data dictionary.
Tablespace    Block Size     Used MB     Free MB    Total MB  Pct. Free        
------------- ---------- ----------- ----------- ----------- ----------        
CWMLITE            4,096           6          14          20         70        
DRSYS              4,096           8          12          20         60        
EXAMPLE            4,096         153           0         153          0        
INDX               4,096           0          25          25        100        
SYSTEM             4,096         241          84         325         26        
TOOLS              4,096           7           3          10         30        
TS_16K            16,384           3           7          10         70        
UNDOTBS            4,096           1         199         200        100        

In the simple example, the SQL subqueries are placed inside the FROM clause and assigned the aliases of df and fs. The df and fs subquery values are then referenced inside the SELECT clause. If you examine this query, you'll see that it sums and compares two ranges of values from two tables, all in a single query.

For some readers, seeing SQL inside the FROM clause is probably quite strange, and the scalar subquery is even stranger! The scalar subquery is a take-off of the in-line view whereby SQL subqueries can be placed inside the SELECT clause. Let's take a look at a few examples.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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