Hi wenhui,
I carefully analyzed the reason for the performance regression with fewer
temporary tables in the previous patch (v1-0002-): the k_hash_funcs determined
by the bloom_create function were 10(MAX_HASH_FUNCS), which led to an excessive
calculation overhead for the bloom filter.
Based on the calculation formula for the bloom filter, when the number of items
is 100 and k_hash_funcs is 2, the false positive rate for a 1KB bloom filter is
0.0006096; when the number of items is 1000, the false positive rate is
0.048929094. Therefore, k_hash_funcs of 2 can already achieve a decent false
positive rate, while effectively reducing the computational overhead of the
bloom filter.
I have re-implemented a bloom_create_v2 function to create a bloom filter with
a specified number of hash functions and specified memory size.
From the test data below, it can be seen that the new patch in the attachment
(v1-0003-) does not lead to performance regression in any scenario.
Furthermore, the default threshold value can be lowered to 2.
Here is the TPS performance data for different numbers of temporary tables
under different thresholds, as compared with the head (98347b5a). The testing
tool used is pgbench, with the workload being to insert into one temporary
table (when the number of temporary tables is 0, the workload is SELECT 1):
|tablenum |0 |1 |2 |5 |10 |100 |1000 |
|--------------|------------|------------|-----------|-----------|-----------|-----------|-----------|
|head(98347b5a)|39912.722209|10064.306268|9183.871298|7452.071689|5641.487369|1073.203851|114.530958 |
|threshold-2 |40097.047974|10009.598155|9982.172866|9955.651235|9999.338901|9785.626296|8278.828828|
Here is the TPS performance data for different numbers of temporary tables
at a threshold of 2, compared with the head (commit 98347b5a). The testing tool
is pgbench, with the workload being to insert into all temporary tables:
|table num |1 |2 | 5 |10 |100 |1000 |
|--------------|-----------|-----------|-----------|-----------|----------|---------|
|head(98347b5a)|7243.945042|5734.545012|3627.290594|2262.594766|297.856756|27.745808|
|threshold-2 |7289.171381|5740.849676|3626.135510|2207.439931|293.145036|27.020953|
I have previously researched the implementation of the Global Temp Table (GTT)
you mentioned, and it have been used in Alibaba Cloud's PolarDB (Link [1]).
GTT can prevent truncation operations on temporary tables that have not been
accessed by the current session (those not in the OnCommitItem List), but GTT
that have been accessed by the current session still need to be truncated at
commit time.Therefore, GTT also require the optimizations mentioned in the
above patch.
[1] https://www.alibabacloud.com/help/en/polardb/polardb-for-oracle/using-global-temporary-tables?spm=a3c0i.23458820.2359477120.1.66e16e9bUpV7cK
