Category Archives: External tables

Thoughts on Big Data SQL SmartScan

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Just a few thoughts I’ve been meaning to blog for a while:

1. Number of columns that can be offloaded

Exadata

Very early on in the Exadata Smart Scan implementation a restriction was added to prevent offloading more than 255 columns. This was done because of performance issues observed by customers. Smart Scan works on 1 MB chunks at a time and needs all the row pieces up to the highest #segcol referenced to be present in the 1 MB chunk that is being processed. When rows with more than 255 columns are first inserted all the row pieces are contiguous and hence likely to be present in the 1 MB chunk.

However, if the DBA has not provided sufficient freespace (PCTFREE) in the block when it is first loaded subsequent DMLs may end up splitting row pieces and moving pieces to blocks in the freelist that have more space available. PCTFREE specifies how much empty space to leave for future updates when a block is first populated.

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Understanding External Table URowids

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I finally found time to get back to External Tables and have a list of blog posts I need to write on this topic. Here’s a brief one.

DBMS_ROWID will nicely break down a heap table’s rowid for you into file number, block number, and row number but it doesn’t handle the rowids coming from External Tables. So let’s look at how to make sense of them. They fall under the datatype UROWID which is a nominally opaque rowid defined by the data source. The first byte of a UROWID tells you which data source it came from and consequently how to deconstruct it.

The easiest way to see what is happening is via the SQL Dump function:

SQL> column xtrowid format a55
SQL> select c_custkey, dump(rowid,16) "XTROWID" from c_et 
  2> where c_custkey < 10;

 C_CUSTKEY XTROWID
---------- -------------------------------------------------------
         1 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,1
         2 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,2
         3 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,3
         4 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,4
         5 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,5
         6 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,6
         7 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,7
         8 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,8
         9 Typ=208 Len=17: 4,0,1,27,a2,0,0,0,0,0,0,0,0,0,0,0,9

9 rows selected.
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External Tables Part 1 – Project Columns All vs Referenced

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I normally blog about table scans on Oracle native data but Oracle also supports a wide variety of features for scanning external tables and I need to cover these too. One of the ways I learn new things is being sent a bug report and saying to myself “Oh! I didn’t know you could do that”. So today I’m going to start with the grammar:

Alter Table <xt> Project Columns [ All | Referenced ]
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Using INMEMORY with External Tables

Posted on by Posted in 12c, External tables, inmemory, oracle, SmartScan Leave a comment

Several people have asked if there is any way to use INMEMORY on External Tables because the INMEMORY syntax is not supported in Create Table and Alter Table with Organization External (actually Create Table parses it but then ignores it).

While there is no way out of having to load the data into Oracle 12.1.0.2, there is a short cut to simplify life when you have an external process recreating flat file data on a regular basis and want to query the latest incarnation against corporate data in Oracle Tablespaces. One example we’ve seen of this is where you have a map-reduce job on Hadoop that highly summarizes large amounts of ephemeral data and it is the summary data that needs to be joined against Oracle data.

Instead of truncating the Oracle table and doing a flat file load each time, we can use a Materialized View on the external table and simply issue an on-demand refresh and let the materialized view take care of the truncate and load under the covers for us.

Since INMEMORY does support Materialized Views, we can now automatically get INMEMORY query speeds on the data from an external source.

Let’s see how this works. For example here’s a loader table I use with TPC-H Scale 1 for local testing:

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