How not to write subqueries:
December 10, 2013
December 8, 2013
It’s finally time to take a close look at the parallel versions of the execution plan I produced a little while ago for a four-table hash join. In this note I’ll examine the broadcast parallel distribution. First, here’s a list of the hints I’m going to use to get the effect I want:
/*+ leading(t4 t1 t2 t3) full(t4) parallel(t4, 2) use_hash(t1) swap_join_inputs(t1) pq_distribute(t1 none broadcast) full(t1) parallel(t1, 2) use_hash(t2) swap_join_inputs(t2) pq_distribute(t2 none broadcast) full(t2) parallel(t2, 2) use_hash(t3) swap_join_inputs(t3) pq_distribute(t3 none broadcast) full(t3) parallel(t3, 2) monitor */
When you upgrade you often find that some little detail (of the optimizer) that didn’t receive a lot of attention in the “New Features” manuals introduces a few dramatic changes in execution plans. Here’s one example of a detail that is likely to catch a few unlucky people. We start with a very simple table which is just and id column with some padding, and then show the effect of a change in the handling of “constant subqueries”. Here’s my data set:
November 29, 2013
A recent question on the OTN database forum included an execution plan that prompted one reader to ask: “but where has the existence subquery gone?” Here’s the original question showing the query, and here’s the later response showing the plan that prompted the question.
There were three possible reasons why that question may have been posed:
November 4, 2013
Here’s the output I got from a 10.2.0.5 system after generating a stored outline on a query – then dropping the index that was referenced by the stored outline and creating an alternative index. Spot the problem:
November 3, 2013
Here’s an odd little optimizer glitch – probably irrelevant to most people, but an indication of the apparent randomness that appears as you combine features. I’ve created an example which is so tiny that the only explanation I can come up with the for optimizer not “behaving properly” is that I’ve found an undocumented restriction relating to a particular feature.
October 18, 2013
Here’s a lovely little example that just came up on the OTN database forum of how things break when features collide. It’s a bug (I haven’t looked for the number) that seems to be fixed in 188.8.131.52. All it takes is a deferrable foreign key and an outer join. I’ve changed the table and column names from the original, and limited the deferability to just the foreign key:
October 14, 2013
Since I’m going to write a couple of articles dissecting parallel execution plans, I thought I’d put up a reference post describing the set of tables I used to generate the plan from the previous post, and the query (with serial execution plan) that produced it. The setup is a simple star schema arrangement – which I’ve generated by created by creating three identical tables and then doing a Cartesian join across the three of them.
October 13, 2013
When you read an execution plan you’re probably trying to identify the steps that Oracle went through to acquire the final result set so that you can decide whether or not there is a more efficient way of getting the same result.
For a serial execution plan this typically means you have to identify the join order, join methods and access methods together with the point at which each predicate was applied. Generally this isn’t too difficult, although subquery pushing (and a couple of presentation bugs relating to other uses of scalar subqueries) can cause a little confusion; and the difference between join order and the order of operation can be slightly obscured when considering hash joins.
Parallel execution plans are harder, though, because you really need to understand the impact of the order of operation, distribution mechanisms chosen, and (in recent versions of Oracle) the timing of the generation and use of Bloom filters. The topic is stunningly large and easy to describe badly; it’s also going to be easy to make generalisations that turn out to be untrue or (at least) sufficiently incomplete as to be misleading. Rather than attempting to cover the topic in one note, I think I’m going to end up writing two or three.
September 27, 2013
Or – to be more accurate – real statistics on a virtual column.
This is one of the “10 top tips” that I came up with for my session with Maria Colgan at OOW13. A method of giving more information that might improve execution plans when you can’t change the code. I’ll start with a small data set including a virtual column (running 184.108.40.206), and a couple of problem queries:
September 25, 2013
(… as in “trick or treat”)
Here’s an important point I learned from Maria Colgan’s “10 tips” presentation on Tuesday of Open World. It comes in two steps – the bit that most people know, and an unexpected consequence:
- autotrace can give misleading execution plans for queries that use bind variables – because autotrace doesn’t peek
- if you run a query after testing it with autotrace, the plan generated by autotrace can be shared by the later execution
Here’s a simple script I’ll be using to demonstrate the behaviour:
September 13, 2013
Here’s a little quiz about Bloom filtering. There seem to be at least three different classes of query where Bloom filters can come into play – all involving hash joins: partition elimination, aggregate reduction on non-mergeable aggregate views, and parallelism.
This quiz is about parallel queries – and all you have to do is work out how many Bloom filters were used in the following two execution plans (produced by 220.127.116.11), and where they were used.
September 7, 2013
I’ve written notes about the different join mechanisms in the past – but such things are always worth revisiting, so here’s an accumulated bundle of comments about hash joins.
A hash join takes two inputs that (in most of the Oracle literature) are referred to as the “build table” and the “probe table”. These rowsources may be extracts from real tables or indexes, or might be result sets from previous joins. Oracle uses the “build table” to build a hash table in memory, consuming and using the rowsource in a single call; it then consumes the “probe table” one row at a time, probing the in-memory hash table to find a match.
Access to the hash table is made efficient by use of a hashing function applied to the join columns – rows with the same value on the join column end up hashing to the same place in the hash table. It is possible for different input values to produce the same hash value (a hash collision) so Oracle still has to check the actual values once it has identified “probable” joins in the hash table. Because the comparison is based on a hashing mechanism, hash joins can only be used for join predicates that are equality predicates.
August 13, 2013
Here’s a little detail about how the optimizer can handle the nvl() function that I hadn’t noticed before (and it goes back to at least 8i). This is running on 18.104.22.168, and table t1 is just all_objects where rownum <= 20000:
August 5, 2013
I’ve posted this note as a quick way of passing on an example prompted by a twitter conversation with Timur and Maria about Bloom filters:
— Jonathan Lewis (@JLOracle) August 5, 2013
The Bloom filter (capital B because it’s named after a person) is not supposed to appear in Oracle plans unless the query is executing in parallel but here’s an example which seems to use a serial Bloom filter. Running in 22.214.171.124 and 126.96.36.199 (the results shown are the latter – the numbers are slightly different between versions):