Everything about Table Scans including offload to Exadata, Big Data SQL, and External Tables

  • August 2, 2017

The beginners guide to Oracle Table Scans

Roger MacNicol
Software Architect

I was asked a question yesterday that reminded me there are always people completely new to the topic who need an introduction  - somewhere to start before the other articles make sense. So, here's my brief write-up of everything you need to know about the basic of Oracle Table Scans.

Oracle has four main ways of scanning a table: the pre-9ir2 table scan, the 9ir2 TurboScan, the Exadata SmartScan, and the In-Memory Scan. Before we summarize each one, the other fundamental piece of information is the Oracle dictum that all blocks much be self-describing: a table scan routine should be able to inspect a block and understand what object it belongs, whether it needs an undo applying, and how the data is laid out without reference to any external structures or secondary storage.

The original table scan routine

Oracle uses a "dataflow" query engine which means a query plan is built from nodes like a sausage machine that have three basic operations: Open, Next, Close. 'Open' means you ask the next node in the chain to prepare to do some work including acquiring any resources it may need, 'Next' means you fetch one unit of work from your child e.g. a row, and 'Close' means to tell your child node to shut down and release any resources it may be holding. You build a query by connecting the right kinds of nodes together in the order you want: one node just sorts, another groups, another does hash joins. The end of the sausage machine is the node seen on query plans as "Table Access Full"

This node would ask the data layer to fetch a block from disk then get rows one at a time from the data layer. This is the work horse table scan: it can scan any kind of data and do SCN manipulations like row versions but it is not the fastest way to scan a table.

9ir2 TurboScan

In 9ir2 we introduced a much faster way of scanning tables called TurboScan. The data layer function which had been handing out rows one at a time was replaced by one that stays in a tight loop retrieving rows from disk and pushing them into a callback supplied by "Table Access Full". An automation tool was used to generate several versions of this routine that optimized out common choices that has to be made: does the user need rowids to be projected? do they need predicates applying? is the data compressed or? is the data column-major or row-major? etc etc Every time a CPU reaches a branch in the code it tries to guess which side of the branch will be taken but if it guess wrong there can be a considerable stall during which no work gets done. By removing most of the branches, the code runs much much more quickly.

TurboScan is used for all queries which do not use RAW datatypes and which do not need special SCN processing.

Both pre-9ir2 scan and TurboScan can use the buffer cache to get blocks (typically small to medium tables) or use Direct Read to get blocks (typically medium to large tables).

See: https://blogs.oracle.com/smartscan-deep-dive/when-bloggers-get-it-wrong-part-1

TurboScan can be disabled for triage purposes by setting:

SQL> alter session set events='12099 trace name context forever, level 1';

or specifically you can disable it only for HCC tables by setting:

SQL> alter session set "_arch_comp_dbg_scan"=1;

Exadata SmartScan

In we introduced Exadata SmartScan intelligent storage. This is where a thin layer of database processing is embedded in the storage cells and the table scan routine offloads simple search criteria and a list of the columns it needs to storage and the storage cells pre-process the blocks to remove rows that fail the search criteria and remove columns which are not needed by the table scan. If all the rows are removed, the block doesn't have to be sent back at all. 

SmartScan can drastically reduce the amount of data returned on the Interconnect and put on the RDBMS memory bus and the space used in SGA by the returned data. An additional significant benefit is gained when the CPU fetches the reduced blocks into the CPU cache since only relevant information exists on the block there is not space wasted by unwanted columns interspersing the wanted columns meaning more relevant data can fit in memory and the CPU prefetch can do a better job of predicting which memory cache line to fetch next.

Only TurboScan Direct Read scans can use this offload capability. You can disable SmartScan for triage purposes by setting:

SQL> alter session set cell_offload_processing=FALSE;
SQL> select /*+ opt_param(‘cell_offload_processing’,’false') */  <col> from <tab> where <predicate>; 

In-Memory Scans

In-Memory scans were introduced in and brought a revolutionary increase in table scan speeds. With In-Memory scans the table or partition is loaded into a in-memory tablespace in SGA known as the inmemory-area. Data is stored in compressed columnar format typically up to 500,000 values in each columnar compression unit. This tablespace is kept transactionally consistent with the data on disk via means of an invalidation bitmap.
Just like with SmartScan, only TurboScan can use In-Memory scans with In-Memory objects. Instead of getting a block from disk, the specialized version of the scan routines fetches a column run from each column of interest, process the search criteria, then returns column runs with the failing rows removed to the "Table Access Full" node.
If any rows have been modified and committed by other users or the users own transaction has modified any rows the scan will see these rows set in the invalidation bitmap. These rows are removed from the columnar results and the additional rows required are fetched from the buffer cache before moving on to the next set of column runs. This works well because the most recently modified blocks are the ones most likely to still be in the buffer cache.




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