Hi to all, the contents present at this web page are really remarkable for people experience, well, keep up the nice work fellows. Hi, can u pls explain y emp is considered as Hash Table whereas it is mentioned the smaller one usually be use as HT. Hi Mohua, Normally a hash join would choose the smaller table to hash.
Save my name, email, and website in this browser for the next time I comment. Toggle navigation. Table 1. HASH join.
Question on hash join. Here is the quote: "If I were to sit down and write the hash-join algorithm, I would have gone in the other direction. I would have hashed the table whose rows were not being preserved, and then read the table whose rows were being preserved, probing the hash table to see whether there was a row to join to.
Oracle has chosen to do it the other way" I think the best way is Oracle should give user a hint similar to that of nested loop to let user be involved in choosing which table used as the outer one, which to be used as the inner one.
For example, in the case of the example you give on Page But the disadvantages of this approach is the order of the resultset is in order of the EMP table. Any advice is very appreciated! August 04, - pm UTC. Currently, in 10g - we can go "either way" and the optimizer will decide which way to go. In 9i - there was ONLY one way - so a hint would be bad since if you told it to do it a way that wasn't implemented The order by happens after the join regardless.
Most useful to me Tom. Except for the following sentence I am still confused. Question about hash join. Hi I encountered this problem recently. Elapsed: I am not worried about elapsed time but I cannot understand why it would pick such plan when nested loops when hinted does the same job with lot less LIOs and elapsed time is less then 4 seconds. I cannot understand what perhaps would confuse the optimizer to give NL a much higher cost 13K which in practise looks like a cheaper operation.
Any pointers would be helpful, Thanks. October 09, - am UTC. This was just what I was looking for! Just the right amount of detail and good examples! Please do a similar presentation on nest loops and merge joins A reader, October 31, - pm UTC. Please do a similar presentation on nest loops and merge joins.
Thanks for a great explanation Tom. For instance, I have a query that hash joins two larger tables A and B and then does a group by. I though that the hash join was performed during the full scan of the larger table B? Or is this a result of a hash table not fitting into memory? November 08, - pm UTC. Hash joins and temp space scarletmanuka, November 19, - pm UTC. Thanks for the info.
Regarding the paging to temp issue, does the CBO look at the amount of data and estimate whether it will fit, or is there a database parameter affecting the ranking between nested loops and hash joins? We have several fairly large tables million records which are all being joined together in a regular report. This report is now failing due to data growth this system has been accumulating data for four months now; statistics are analysed regularly.
Oracle is running out of temp space trying to hash join everything together, even though the tables are joined by the primary key indexes. I can get around this by adding selected index hints, but I wondered if there was a better way.
November 20, - am UTC. Hash joins and temp space scarletmanuka, November 20, - pm UTC. Sorry, should have been clearer in my example. So the hash joins are spending a lot of time and running out of temp space going over rows we don't actually need. Adding more useful indexes to the database would certainly help too, and would probably help the CBO avoid the hash joins that are killing it, but I have to go and nudge the DBA a bit more to get that done.
The link from the main table to the period table is indexed, but the field on the period table that we're using to restrict this query is not indexed!! So the CBO doesn't realise that we don't need all the data So I'm not blaming the CBO, it's just that until the indexing is fixed I need to persuade it not to do the right thing.
November 22, - pm UTC. Most brilliant! Apurva, November 26, - pm UTC. Tom, You are a magician! November 26, - pm UTC. Tom, Your demo referenced in the tinyurl doesn't work right now. Can you put in a new URL? December 02, - pm UTC. These are really very good discussions that educate us way beyond any Book or manual. December 04, - am UTC. I'm writing here becouse I hope you bring some ligth in a problem related to which factors the optimizer take into account when it choose "the best plan" with hash or with loops.
I read all the infos presented about hash join and I could't find an acceptable answer. So, here is the situation : I have a table 'pricelists' desc pricelist Name Null?
The same table structure an indexes is define for two diferent users schema on the same server 9. The only diference is in the number of rows : one have and the other ID, pl1. Of course, the fast one is the one on the bigger table with the hash join no matter what the optimeizer said. What I don't understand is WHY? As I said the only difference is in the number of rows so I delete the statistics for the smaller table and suprise : the execution plan was the one with hash join!
So, how is oracle choosing hash join or nested loop based only on the number of rows or better said only on statistics because all other parameters are the same??? Below I collected the time for different volume of records in staging table which is hashed. But when it is increased to I see a big difference of Is this because the staging table is unable to fit in memory. How can I overcome this problem? Do I have to increase the memory size?
If so how to do that? November 16, - pm UTC. You don't "have" to do anything, you should work with your DBA as you are probably NOT the only thing on this machine in real life and hence consuming all memory for you would not be good - it needs to be done in coordination with the system as a whole. The DBA or administrator of this system can advise you on how you might set your hash area size for your purposes.
Follow up karthick pattabiraman, November 13, - am UTC. This is a follow-up to my previous posting. After doing some research on hash join i came up with this. Just thought of sharing with everyone. From the performance tuning guide "Ideally, the size of a work area is big enough that it can accommodate the input data and auxiliary memory structures allocated by its associated SQL operator. This is known as the optimal size of a work area.
How can I determine if the table or the result set that is been hashed fits in the Hash area. How to find the size of the result set or the table to determine the optimal size of the work area. In 10g, when set to auto, auto is used in dedicated and shared servers. I found the answer :o karthick pattabiraman, November 13, - am UTC. Found all the answers by my self.
Tom correct me if iam wrong. ANS: This is answered in first question itself 3. Got this beautiful query from the doc.. Any way after changing the size it need to be tested. This worked for me Hi Tom, I am stuck with a puzzle and have spent a lot of time but have no clue about it so far.
I have an application back ground and it's only this incident that I will talk about which lead me to dwell deep into Oracle. I was not satisfied with the fact that we disabled hash joins all together. There has to be a reason why Oracle made this parameter obsolete. If it is obsolete why use it? None of the DBAs had any satisfying answer.
This lead me to pursue an answer myself and that's when I got your books and jumped into Oracle. I have been trying all sorts of experiments to find an answer. Last 3 weeks have been spent in search of enlightenment but in vain :.
This forum is my last hope! I have failed to understand the rationale behind what improved the performance. Why were hash joins detrimental to the performance? And if they were why did not optimizer chose sort merge at the first place? Here are some facts. Oracle version is The memory management is automatic and the aggregate PGA is set to 1. The Target SGA is 3. When hash joins are disabled the optimizer does sort merges mostly.
With hash joins the LIO's are almost times more. The LIO for the queries is pretty high. The statistics are up to date. The tables in concern range from 1M to 20M rows. Any help or any lead would really be appreciated. Regards, Saurabh. Hello, I face similar issues as Saurabh Khurana. I can not figure out why explain plan in However, I can not figure why I do not put specific query here, I have seen these behaviour on XX databases on many machines with many different SQLs just one thing is common for that issue to get optimizer in wrong way: "OR" is used in such queries Any hint?
January 30, - pm UTC. And provide whether you intended to get the first rows as fast as possible or whether you wanted to get all rows. Hash join is most certainly "not a dummy approach" in any way shape or form.
My guess is either a you have some wickedly poor init. That query there - it would take seconds to return the first record using the hash join, but 10 seconds to get all of them. Reader, February 03, - pm UTC. Tom, Are there any ohter prsentations like the one below? In particular, for the left-deep join tree we have:.
Bushy joins yes, they are really called that or zigzag join trees have some of the row sources swapped but not all as in the case of left-deep and right-deep join trees. To be specific, we obtain that particular join order as indicated:. Interestingly, bushy joins are never considered by the optimizer.
Hence, if you believe a bushy join to be the best join order possible, you have to force Oracle with the leading hint. Each parallel query server reads data from a particular partition of the first table and joins it with the appropriate rows from the corresponding partition of the second table. Query servers have no need to communicate to one another, which is ideal.
The only downside is if there is at least one partition that is significantly larger than all the others, as this may affect the balancing of the load.
When only one table is partitioned, Oracle can go with a parallel partial partition-wise join. It re partitions the other table on the fly based on the partitioning scheme of the reference table. Once the partitioning is out of the way, the database proceeds as it does with a full partition-wise join. It is generally recommended to use hash instead of range partitioning for partition-wise joins to be effective, mainly because of possible data skew that leads to some partitions being larger than others.
Furthermore, the number of partitions in relation to the DOP is relevant to the performance. Ideally, the number of partitions is a multiple of the number of query servers. Both hash and sort-merge joins are possible for full partition-wise joins. Another gotcha with hash joins is that they can only be used with equality join conditions. Join T1 and T2. Call the intermediate result set J
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