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mariadb/Docs/optimizer_costs.txt
Shivam Choudhary 6c3b1dced4 Fixed some typos in optimizer_costs.txt
2023-03-28 18:41:09 +11:00

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This file is intended to explain some of the optimizer cost variables
in MariaDB 11.0
Background
==========
Most timings has come from running:
./check_costs.pl --rows=1000000 --socket=/tmp/mysql-dbug.sock --comment="--aria-pagecache-buffer-size=10G --innodb-buffer_pool_size=10G --key_buffer-size=1G --max-heap-table-size=10G"
The MariaDB server is started with the options:
--aria-pagecache-buffer-size=10G --innodb-buffer_pool_size=10G --key_buffer-size=1G --max-heap-table-size=10G"
- All costs are changed to be milliseconds for engine operations and
other calculations, like the WHERE clause. This is a big change from
before the patch that added this file where the basic cost was a
disk seek and one index read and we assumed they had the same cost.
- I am using Aria as the 'base' cost. This is because it caches all data,
which most other engines also would do.
- MyISAM cannot be used as 'base' as it does not cache row data (which gives
a high overhead when doing row lookups).
- Heap is in memory and a bit too special (no caching).
- InnoDB is a clustered engine where secondary indexes has to use
the clustered index to find a row (not a common case among storage engines).
The old assumption in the optimizer has 'always' been that
1 cost = 1 seek = 1 index = 1 row lookup = 0.10ms.
However 1 seek != 1 index or row look and this has not been reflected in
most other cost.
This document is the base of changing things so that 1 cost = 1ms.
Setup
=====
All timings are calculated based on result from this computer:
CPU: Intel(R) Xeon(R) W-2295 CPU @ 3.00GHz
Memory: 256G
Disk: Samsum SSD 860 (not really relevant in this case)
Rows in tests: 1M Each test is run 3 times
(one test to cache the data and 2 runs of which we take the average).
The assumption is that other computers will have somewhat proportional
timings. The timings are done with all data in memory (except MyISAM rows).
This is reflected in the costs for the test by setting
optimizer_disk_read_ratio=0.
Note that even on a single Linux computer without any notable tasks
the run time vary a bit from run to run (up to 4%), so the numbers in
this document cannot be repeated exactly but should be good enough for
the optimizer.
Timings for disk accesses on other system can be changed by setting
optimizer_disk_read_cost (usec / 4092 bytes) to match the read speed.
Default values for check_costs.pl:
optimizer_disk_read_ratio= 0 Everything is cached
SCAN_LOOKUP_COST=1 Cost modifier for scan (for end user)
set @@optimizer_switch='index_condition_pushdown=off'";
ROW_COPY_COST and KEY_COPY_COST
===============================
Regarding ROW_COPY_COST:
When calulating cost of fetching a row, we have two alternativ cost
parts (in addition to other costs):
scanning: rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST)
rnd_pos: rows * (ROW_LOOKUP_COST + ROW_COPY_COST)
In theory we could remove ROW_COPY_COST and just move the cost
to the two other variables. However, in the future there may reason
to be able to modif row_copy_cost per table depending on number and type
of fields (A table of 1000 fields should have a higher row copy cost than
a table with 1 field). Because of this, I prefer to keep ROW_COPY_COST
around for now.
Regarding KEY_COPY_COST:
When calulating cost of fetching a key we have as part of the cost:
keyread_time: rows * KEY_COPY_COST + ranges * KEY_LOOKUP_COST +
(rows-ranges) * KEY_NEXT_FIND_COST
key_scan_time: rows * (KEY_NEXT_FIND_COST + KEY_COPY_COST)
We could remove KEY_COPY_COST by adding it to KEY_LOOKUP_COST and
KEY_NEXT_FIND_COST but I prefer to keep it with the same argument as
for ROW_COPY_COST.
The reation between KEY_COPY_COST / (KEY_NEXT_FIND_COST + KEY_COPY_COST)
is assumed to be 0.1577 (See analyze in the appendix)
There is a relationship between the above costs in that for a clustered
index the cost is calculated as ha_keyread_time() + ROW_COPY_COST.
Preramble
=========
I tried first to use performance schema to get costs, but I was not
successful as all timings I got for tables showed the total time
executing the statement, not the timing for doing the actual reads.
Also the overhead of performance schema affected the results
With --performance-schema=on
MariaDB [test]> select sum(1) from seq_1_to_100000000;
+-----------+
| sum(1) |
+-----------+
| 100000000 |
+-----------+
1 row in set (4.950 sec)
Performance schema overhead: 30.1%
With:
UPDATE performance_schema.setup_consumers SET ENABLED = 'YES';
UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES';
Flush with:
CALL sys.ps_truncate_all_tables(FALSE);
Performance schema overhead now: 32.9%
Timings from:
select * from events_statements_current where thread_id=80;
MariaDB [test]> select 885402302809000-884884140290000;
+---------------------------------+
| 885402302809000-884884140290000 |
+---------------------------------+
| 518162519000 |
+---------------------------------+
-> Need to divide by 1000000000000.0 to get seconds
As seen above, the above gives the total statement time not the time
spent to access the tables.
In the end, I decided to use analyze to find out the cost of the table
actions:
For example: Finding out table scan timing (and thus costs):
analyze format=json select sum(1) from seq_1_to_100000000;
r_table_time_ms": 1189.239022
Calculating 'optimizer_where_cost'
==================================
To make the WHERE cost reasonable (not too low) we are assuming there is
2 simple conditions in the default 'WHERE clause'
MariaDB [test]> select benchmark(100000000,l_commitDate >= '2000-01-01' and l_tax >= 0.0) from test.check_costs limit 1;
+--------------------------------------------------------------------+
| benchmark(100000000,l_commitDate >= '2000-01-01' and l_tax >= 0.0) |
+--------------------------------------------------------------------+
| 0 |
+--------------------------------------------------------------------+
1 row in set (3.198 sec)
Time of where in seconds: 3.198 / 100000000 (100,000,000)
Verification:
select sum(1) from seq_1_to_100000000 where seq>=0.0 and seq>=-1.0;
+-----------+
| sum(1) |
+-----------+
| 100000000 |
+-----------+
1 row in set (8.564 sec)
MariaDB [test]> select sum(1) from seq_1_to_100000000;
+-----------+
| sum(1) |
+-----------+
| 100000000 |
+-----------+
1 row in set (5.162 sec)
Time of where= (8.564-5.162)/100000000 = 3.402/100000000 (100,000,000)
(Result good enough, as sligthly different computations)
check_costs.pl comes provides the numbers when using heap tables and 1M rows:
simple where: 118.689 ms
complex where: 138.474 ms
no where: 83.699 ms
Which gives for simple where:
(118.689-83.699)/1000 = 0.034990000000000007 ms
Which is in the same ballpark.
We use the result from the select benchmark run as this has least overhead
and is easiest to repeat and verify in a test.
Which gives:
optimizer_where_cost= 0.032 ms / WHERE.
HEAP TABLE SCAN & ROW_COPY_COST
===============================
We start with heap as all rows are in memory and we don't have to take
disk reads into account.
select sum(l_partkey) from test.check_costs
table_scan ms: 10.02078736
rows: 1000000
Cost should be 10.02078736 (scan cost) + 32 (where cost)
cost= scan_time() * optimizer_cache_cost * SCAN_LOOKUP_COST +
TABLE_SCAN_SETUP_COST +
records * (ROW_COPY_COST + ROW_LOOKUP_COST + WHERE_COMPARE_COST);
=>
We are ignoring TABLE_SCAN_SETUP (which is just to prefer index lookup on small
tables).
We can also ignore records * WHERE_COMPARE_COST as we don't have that
in the above calcuated 'ms'.
row_costs= (ROW_COPY_COST + ROW_LOOKUP_COST)
cost= scan_time() * 1 * 1 +
1000000.0 * (row_costs)
=>
cost= time_per_row*1000000 + row_costs * 1000000;
=>
time_per_row+row_cost= cost/1000000
Let's assume that for heap, finding the next row is 80 % of the time and
copying the row (a memcmp) to upper level is then 20 %.
(This is not really important, we could put everthing in heap_scan_time,
but it's good to have split the data as it gives us more options to
experiment later).
row_lookup_cost= 10.02078736/1000000*0.8 = 8.0166298880000005e-06
row_copy_cost= 10.02078736/1000000*0.2 = 2.0041574720000001e-06
Conclusion:
heap_scan_time= 8.0166e-06
row_copy_cost= 2.0042e-06
Heap doesn't support key only read, so key_copy_cost is not relevant for it.
HEAP INDEX SCAN
===============
select count(*) from test.check_costs_heap force index (l_suppkey) where l_suppkey >= 0 and l_partkey >=0
index_scan time: 79.7286117 ms
Index scan on heap tables can only happen with binary trees.
l_supp_key is using a binary tree.
cost= (ranges + rows + 1) * BTREE_KEY_NEXT_FIND_COST + rows * row_copy_cost=
(for large number of rows):
rows * (BTREE_KEY_NEXT_FIND_COST + row_copy_cost)
BTREE_KEY_NEXT_FIND_COST= cost/rows - row_copy_cost =
79.7286117/1000000- 2.334e-06= 0.0000773946117
HEAP EQ_REF
===========
select straight_join count(*) from seq_1_to_1000000,test.check_costs_heap where seq=l_linenumber
eq_ref_index_join time: 175.874165 of which 12.57 is from seq_1_to_1000000
Note: This is 34% of the cost of an Aria table with index lookup and
20% of an Aria table with full key+row lookup.
cost= rows * (key_lookup_cost + row_copy_cost)
key_lookup_cost= cost/rows - key_copy_cost =
(175.874165-12.57)/1000000 - 2.334e-06 = 0.00016097016500000002
HEAP EQ_REF on binary tree index
================================
select straight_join count(*) from seq_1_to_1000000,test.check_costs_heap where seq=l_extra and l_partkey >= 0
eq_ref_join time: 241.350539 ms of which 12.57 is from seq_1_to_1000000
rows * (tree_find_cost() + row_copy_cost) =
tree_find_cost()= cost/rows - row_copy_cost =
(241.350539-12.57)/1000000 - 2.334e-06= 0.000226446539
tree_find_cost() is defined as key_compare_cost * log2(table_rows)
->
key_compare_cost= 0.000226446539/log2(1000000) = 0.000011361200108882259;
SEQUENCE SCAN
=============
analyze format=json select sum(seq+1) from seq_1_to_1000000;
r_table_time_ms: 12.47830611
Note that for sequence index and table scan is the same thing.
We need to have a row_copy/key_copy cost as this is used when doing
an key lookup for sequence. Setting these to 50% of the full cost
should be sufficient for now.
Calculation sequence_scan_cost:
When ignoring reading from this, the cost of table scan is:
rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST)
The cost of key scan is:
ranges * KEY_LOOKUP_COST + (rows - ranges) * KEY_NEXT_FIND_COST +
rows * KEY_COPY_COST;
As there is no search after first key for sequence, we can set
KEY_LOOKUP_COST = KEY_NEXT_FIND_COST.
This gives us:
r_table_time_ms = (ROW_NEXT_FIND_COST + ROW_COPY_COST) =
(KEY_NEXT_FIND_COST + KEY_COPY_COST) * 1000000;
->
ROW_NEXT_FIND_COST= ROW_COPY_COST = KEY_LOOKUP_COST + KEY_COPY_COST=
12.47830611/1000000/2 = 0.0000062391530550
HEAP KEY LOOKUP
===============
We can use this code to find the timings of a index read in a table:
analyze format=json select straight_join count(*) from seq_1_to_1000000,check_costs where seq=l_orderkey
"query_block": {
"select_id": 1,
"r_loops": 1,
"r_total_time_ms": 420.5083447,
"table": {
"table_name": "seq_1_to_1000000",
"access_type": "index",
"possible_keys": ["PRIMARY"],
"key": "PRIMARY",
"key_length": "8",
"used_key_parts": ["seq"],
"r_loops": 1,
"rows": 1000000,
"r_rows": 1000000,
"r_table_time_ms": 12.47830611,
"r_other_time_ms": 44.0671283,
"filtered": 100,
"r_filtered": 100,
"using_index": true
},
"table": {
"table_name": "check_costs",
"access_type": "eq_ref",
"possible_keys": ["PRIMARY"],
"key": "PRIMARY",
"key_length": "4",
"used_key_parts": ["l_orderkey"],
"ref": ["test.seq_1_to_1000000.seq"],
"r_loops": 1000000,
"rows": 1,
"r_rows": 1,
"r_table_time_ms": 160
"filtered": 100,
"r_filtered": 100,
"attached_condition": "seq_1_to_1000000.seq = check_costs.l_orderkey"
}
}
This gives the time for a key lookup on hash key as:
160/10000000 - row_copy_cost =
160/1000000.0 - 2.0042e-06 = 0.00015799580000000002
ARIA TABLE SCAN
===============
(page format, all rows are cached)
table_scan ms: 107.315698
Cost is calculated as:
blocks= stats.data_file_length / stats.block_size) = 122888192/4096= 30002
engine_blocks (8192 is block size in Aria) = 15001
cost= blocks * avg_io_cost() *
optimizer_cache_cost * SCAN_LOOKUP_COST +
engine_blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
records * (ROW_NEXT_FIND_COST + ROW_COPY_COST));
When all is in memory (optimizer_cache_cost= 0) we get:
cost= blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
records * (ROW_NEXT_FIND_COST + ROW_COPY_COST));
To calculate INDEX_BLOCK_COPY_COST I added a temporary tracker in
ma_pagecache.cc::pagecache_read() and did run the same query.
I got the following data:
{counter = 17755, sum = 1890559}
Which give me the time for copying a block to:
1000.0*1890559/sys_timer_info.cycles.frequency/17755 = 3.558138826971332e-05 ms
And thus INDEX_BLOCK_COPY_COST= 0.035600
Replacing known constants (and ignore TABLE_SCAN_SETUP_COST):
cost= 107.315698 = 15001 * 3.56e-5 + 1000000 * aria_row_copy_costs;
aria_row_copy_costs= (107.315698 - (15001 * 3.56e-5))/1000000 =
0.0001067816624
As ROW_COPY_COST/ROW_NEXT_FIND_COST= 0.57 (See appendex)
ROW_COPY_COST= 0.0001067816624 * 0.57 = 0.000060865547560
ROW_NEXT_FIND_COST= 0.0001067816624 * 0.43 = 0.000045916114832
Aria, INDEX SCAN
================
Finding out cost of reading X keys from an index (no row lookup) in Aria.
Query: select count(*) from test.check_costs_aria force index (l_suppkey) where l_suppkey >= 0 and l_partkey >=0
Table access time: ms: 98.1427158
blocks= index_size/IO_SIZE =
(rows * tot_key_length / INDEX_BLOCK_FILL_FACTOR) / IO_SIZE
->
1000000 * 19 / 0.75/ 4096 = 6184
engine_blocks (block_size 8192) = 6184/2 = 3092
(Range optimzer had calculated 3085)
keyread_time= blocks * avg_io_cost() * cache + engine_blocks * INDEX_BLOCK_COPY_COST + rows * (KEY_NEXT_FIND_COST + KEY_COPY_COST);
= engine_blocks * INDEX_BLOCK_COPY_COST + rows * KEY_NEXT_FIND_COST=
3092 * 3.56e-05 + 1000000 * (KEY_NEXT_FIND_COST + KEY_COPY_COST)
->
KEY_NEXT_FIND_COST + KEY_COPY_COST= (98.1427158 - 3092 * 3.56e-05)/1000000 =
0.0000980326406;
KEY_COPY_COST= 0.0000980326406 * 0.16 = 0.000015685222496
KEY_NEXT_FIND_COST= 0.0000980326406 * 0.84 = 0.000082347418104
Aria, RANGE SCAN (scan index, fetch a row for each index entry)
===============================================================
Query:
select sum(l_orderkey) from test.check_costs_aria force index(l_suppkey) where l_suppkey >= 0 and l_partkey >=0
range_scan ms: 309.7620909
cost= keyread_time + rnd_pos_time.
keyread_time is as above in index scan, but whithout KEY_COPY_COST:
keyread_time= 98.1427158 - KEY_COPY_COST * 1000000=
98.1427158 - 0.000015685222496 * 1000000= 82.457493304000000;
rnd_pos_time= 309.7620909 - 82.457493304000000 = 227.304597596000000
rnd_pos_time() = io_cost + engine_mem_cost +
rows * (ROW_LOOKUP_COST + ROW_COPY_COST) =
rows * avg_io_cost() * engine_block_size/IO_SIZE +
rows * INDEX_BLOCK_COPY_COST +
rows * (ROW_COPY_COST + ROW_LOOKUP_COST)
= (When rows are in memory)
rows * INDEX_BLOCK_COPY_COST +
rows * (ROW_COPY_COST + ROW_LOOKUP_COST)
This gives us:
227.304597596000000 = 1000000 * 3.56e-05 + 1000000*(0.000060865547560 + ROW_LOOKUP_COST)
->
ROW_LOOKUP_COST= (227.304597596000000 - 1000000 * 3.56e-05 - 1000000*0.000060865547560) / 1000000 = 0.0001308390500
Aria, EQ_REF with index_read
============================
select straight_join count(*) from seq_1_to_1000000,test.check_costs_aria where seq=l_linenumber
eq_ref_index_join 499.631749 ms
According to analyze statement:
- Cost for SELECT * from seq_1_to_1000000: 12.57
(From Last_query_cost after the above costs has been applied)
- Time from check_costs: eq_ref's: 499.631749- 12.57s = 487.061749
cost= rows * (keyread_time(1,1) + KEY_COPY_COST)
keyread_time(1,1)= INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST;
cost= rows * (KEY_COPY_COST + INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST)
->
KEY_LOOKUP_COST= cost/rows - 0.000015685222496 - 0.000035600
KEY_LOOKUP_COST= 487.061749 / 1000000 - 0.000035600 - 0.000015685222496
KEY_LOOKUP_COST= 0.000435776526504
MyISAM, TABLE SCAN
==================
select sum(l_partkey) from test.check_costs_myisam
table_scan ms: 126.353364
check_costs.MYD: 109199788 = 26660 IO_SIZE blocks
The row format for MyISAM is similar to Aria, so we use the same
ROW_COPY_COST for Aria.
cost= blocks * avg_io_cost() *
optimizer_cache_cost * SCAN_LOOKUP_COST +
engine_blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST));
MyISAM is using the file system as a row cache.
Let's put the cost of accessing the row in ROW_NEXT_FIND_COST.
Everything is cached (by the file system) and optimizer_cache_cost= 0;
cost= engine_blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST))
ROW_NEXT_FIND_COST=
(costs - engine_blocks * INDEX_BLOCK_COPY_COST - TABLE_SCAN_SETUP_COST)/rows -
ROW_COPY_COST
=
(126.353364 - 26660 * 3.56e-05 - 1)/1000000 - 0.000060865547560
ROW_NEXT_FIND_COST= 0.00006353872044
MyISAM INDEX SCAN
=================
select count(*) from test.check_costs_myisam force index (l_suppkey) where l_suppkey >= 0 and l_partkey >=0;
index_scan ms: 106.490584
blocks= index_size/IO_SIZE =
(rows * tot_key_length / INDEX_BLOCK_FILL_FACTOR) / IO_SIZE
->
1000000 * 19 / 0.75/ 4096 = 6184
As MyISAM has a block size of 4096 for this table, engine_blocks= 6184
cost= keyread_time= blocks * avg_io_cost() * cache + engine_blocks * INDEX_BLOCK_COPY_COST + rows * (KEY_NEXT_FIND_COST + KEY_COPY_COST);
->
cost= engine_blocks * INDEX_BLOCK_COPY_COST + rows * KEY_NEXT_FIND_COST
Assuming INDEX_BLOCK_COPY_COST is same as in Aria and the code for
key_copy is identical to Aria:
cost= 6184 * 3.56e-05 + 1000000 * (KEY_NEXT_FIND_COST + KEY_COPY_COST)
->
KEY_NEXT_FIND_COST= (106.490584 - 6184 * 3.56e-05)/1000000 - 0.000015685222496=
0.000090585211104
MyISAM, RANGE SCAN (scan index, fetch a row for each index entry)
=================================================================
select sum(l_orderkey) from test.check_costs_myisam force index(l_suppkey) where l_suppkey >= 0 and l_partkey >=0 and l_discount>=0.0
time: 1202.0894 ms
cost= keyread_time + rnd_pos_time.
keyread_time is as above in MyISAM INDEX SCAN, but without KEY_COPY_COST:
keyread_time= 106.490584 - KEY_COPY_COST * 1000000=
106.490584 - 0.000015685222496 * 1000000= 90.805361504000000;
rnd_pos_time= 1202.0894 - 90.805361504000000 = 1111.284038496000000
rnd_pos_time() = io_cost + engine_mem_cost +
rows * (ROW_LOOKUP_COST + ROW_COPY_COST) =
rows * avg_io_cost() * engine_block_size/IO_SIZE +
rows * INDEX_BLOCK_COPY_COST +
rows * (ROW_COPY_COST + ROW_LOOKUP_COST)
= (When rows are in memory)
rows * INDEX_BLOCK_COPY_COST +
rows * (ROW_COPY_COST + ROW_LOOKUP_COST)
This gives us:
1111.284038496000000 = 1000000 * 3.56e-05 + 1000000*(0.000060865547560 + ROW_LOOKUP_COST)
->
ROW_LOOKUP_COST= ( 1111.284038496000000 - 1000000 * (3.56e-05 + 0.000060865547560)) / 1000000s
->
ROW_LOOKUP_COST= 0.001014818490936
As the row is never cached, we have to ensure that rnd_pos_time()
doesn't include an io cost (which would be affected by
optimizer_cache_hit_ratio). This is done by having a special
ha_myisam::rnd_pos_time() that doesn't include io cost but instead an
extra cpu cost.
MyISAM, EQ_REF with index_read
==============================
select straight_join count(*) from seq_1_to_1000000,test.check_costs_myisam where seq=l_linenumber;
eq_ref_join ms: 613.906777 of which 12.48 ms is for seq_1_to_1000000;
According to analyze statement:
- Cost for SELECT * from seq_1_to_1000000: 12.48 (See sequence_scan_cost)
- Time from check_costs: eq_ref's: 613.906777- 12.48 = 601.426777;
cost= rows * (keyread_time(1) + KEY_COPY_COST)
keyread_time(1)= INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST;
cost= rows * (KEY_COPY_COST + INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST)
->
KEY_LOOKUP_COST= cost/rows - INDEX_BLOCK_COPY_COST - KEY_COPY_COST;
601.426777 / 1000000 - 3.56e-05 - 0.000015685222496 = 0.00055014155451
KEY_LOOKUP_COST= 0.00055014155451
InnoDB, TABLE SCAN
==================
select sum(l_quantity) from check_costs_innodb;
table_scan 131.302492
Note that InnoDB reported only 956356 rows instead of 100000 in stats.records
This will will cause the optimizer to calculate the costs based on wrong
assumptions.
As InnoDB have a clustered index (which cost is a combination of
KEY_LOOKUP_COST + ROW_COPY_COST), we have to ensure that the
relationship between KEY_COPY_COST and ROW_COPY_COST is close to the
real time of copying a key and a row.
I assume, for now, that the row format for InnoDB is not that
different than for Aria (in other words, computation to unpack is
about the same), so lets use the same ROW_COPY_COST (0.000060865547560)
I am ignoring the fact that InnoDB can optimize row copying by only
copying the used fields as the optimizer currently have to take that
into account. (This would require a way to update ROW_COPY_COST /
table instance in the query).
For now, lets also use the same value as Aria for
INDEX_BLOCK_COPY_COST (3.56e-05).
The number of IO_SIZE blocks in the InnoDB data file is 34728 (from gdb))
(For reference, MyISAM was using 26660 and Aria 30002 blocks)
As InnoDB is using 16K blocks, the number of engine blocks= 34728/4= 8682
cost= blocks * avg_io_cost() *
optimizer_cache_cost * SCAN_LOOKUP_COST +
engine_blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST));
as optimizer_cache_cost = 0
cost= engine_blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST))
ROW_NEXT_FIND_COST=
(costs - engine_blocks * INDEX_BLOCK_COPY_COST - TABLE_SCAN_SETUP_COST)/rows -
ROW_COPY_COST
= (Ignoring TABLE_SCAN_SETUP_COST, which is just 10 usec)
(131.302492 - 8682 * 3.56e-05)/1000000 - 0.000060865547560 =
0.00007012786523999997
InnoDB INDEX SCAN
=================
select count(*) from check_costs_innodb force index (l_suppkey) where l_suppkey >= 0 and l_partkey >=0;
index_scan 114.733037 ms
Note that InnoDB is reporting 988768 rows instead of 1000000
(The number varies a bit between runs. At another run I got 956356 rows)
With default costs (as of above), we get a query cost of 112.142. This can
still be improved a bit...
blocks= index_size/IO_SIZE =
(rows * tot_key_length / INDEX_BLOCK_FILL_FACTOR) / IO_SIZE
-> (total_key_length is 17 in InnoDB, 19 in Aria)
1000000 * 17 / 0.75/ 4096 = 5533
engine_blocks= 5533/4 = 1383
(In reality we get 5293 blocks and 1323 engine blocks, because of the
difference in InnoDB row count)
cost= keyread_time= blocks * avg_io_cost() * cache + engine_blocks * INDEX_BLOCK_COPY_COST + rows * (KEY_NEXT_FIND_COST + KEY_COPY_COST);
->
cost= engine_blocks * INDEX_BLOCK_COPY_COST + rows * KEY_NEXT_FIND_COST
Assuming INDEX_BLOCK_COPY_COST is same as in Aria:
(Should probably be a bit higher as block_size in InnoDB is 16384
compared to 8192 in Aria)
cost= 1383 * 3.56e-05 + 1000000 * (KEY_NEXT_FIND_COST + KEY_COPY_COST)
=
KEY_NEXT_FIND_COST + KEY_COPY_COST= (114.733037 - 1383 * 3.56e-05)/1000000
=
KEY_NEXT_FIND_COST= (114.733037 - 1383 * 3.56e-05)/1000000 - 0.000015685222496
->
KEY_NEXT_FIND_COST=0.000098998579704;
Setting this makes InnoDB calculate the cost to 113.077711 (With estimate of
988768 rows)
If we would have the right number of rows in ha_key_scan_time, we would
have got a cost of:
Last_query_cost: 145.077711 (Including WHERE cost for 988768 row)
(145.077711)/988768*1000000.0-32 = 114.72573444933
InnoDB RANGE SCAN
=================
select sum(l_orderkey) from check_costs_innodb force index(l_suppkey) where l_suppkey >= 0 and l_partkey >=0 and l_discount>=0.0
range_scan 961.4857045 ms
Note that InnoDB was reporting 495340 rows instead of 1000000 !
I added a patch to fix this and now InnoDB reports 990144 rows
cost= keyread_time + rnd_pos_time.
keyread_time is as above in index scan, but we want it without KEY_COPY_COST:
keyread_time= cost - KEY_COPY_COST * 1000000=
114.733037 - 0.000015685222496 * 1000000= 99.047814504000000
rnd_pos_time= 961.4857045 - 99.047814504000000 = 862.437889996000000
rnd_pos_time() = io_cost + engine_mem_cost +
rows * (ROW_LOOKUP_COST + ROW_COPY_COST) =
rows * avg_io_cost() * engine_block_size/IO_SIZE +
rows * INDEX_BLOCK_COPY_COST +
rows * (ROW_COPY_COST + ROW_LOOKUP_COST)
= (When rows are in memory)
rows * (INDEX_BLOCK_COPY_COST + ROW_COPY_COST + ROW_LOOKUP_COST)
This gives us:
862.437889996000000 = 1000000 * 3.56e-05 + 1000000*(0.000060865547560 + ROW_LOOKUP_COST)
->
ROW_LOOKUP_COST= (862.437889996000000 - 1000000*(3.56e-05+0.000060865547560)) / 1000000
->
ROW_LOOKUP_COST= 0.000765972342436
Setting this makes InnoDB calculate the cost to 961.081050 (good enough)
InnodDB EQ_REF with index_read
==============================
select straight_join count(*) from seq_1_to_1000000,test.check_costs_innodb where seq=l_linenumber
time: 854.980610 ms
Here the engine first has to do a key lookup and copy the key to the upper
level (Index only read).
According to analyze statement:
- Cost for SELECT * from seq_1_to_1000000: 12.57 (See sequence_scan_cost)
- Time from check_costs: eq_ref_join: 854.980610
This is time for accessing both seq_1_to_1000000 and check_costs
time for check_cost_innodb: 854.980610-12.57 = 842.410610 ms
cost= rows * (keyread_time(1,1) + KEY_COPY_COST)
keyread_time(1,1)= INDEX_BLOCK_COPY_COST + ranges * KEY_LOOKUP_COST +
(rows-ranges) * KEY_NEXT_FIND_COST
As rows=1 and ranges=1:
keyread_time(1,1)= INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST
cost= rows * (KEY_COPY_COST + INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST)
->
KEY_LOOKUP_COST= cost/rows - INDEX_BLOCK_COPY_COST - KEY_COPY_COST;
842.410610 / 1000000 - 3.56e-05 - 0.000015685222496
->
KEY_LOOKUP_COST= 0.000791125387504;
After the above we have
last_query_cost=918.986438;
The cost for check_costs_innodb =
last_query_cost - sequence_scan_cost - where_cost*2 =
918.986438 - 12.57 - 32*2 = 842.416438 (ok)
InnodDB EQ_REF with clustered index read
========================================
select straight_join count(*) from seq_1_to_1000000,check_costs_innodb where seq=l_orderkey
eq_ref_cluster_join time: 972.290773 ms
According to analyze statement:
- Cost for SELECT * from seq_1_to_1000000: 12.57 (See sequence_scan_cost)
- Time from check_costs: eq_ref_cluster_join: 972.290773 ms
This is time for accessing both seq_1_to_1000000 and check_costs_innodb.
Time for check_cost_innodb: 972.290773 - 12.57 = 959.790773
The estimated cost is 875.0160
cost= rows * (keyread_time(1,1) +
ranges * ROW_LOOKUP_COST +
(rows - ranges) * ROW_NEXT_FIND_COST +
rows * ROW_COPY_COST)
As rows=1 and ranges=1:
cost= rows * (INDEX_BLOCK_COPY_COST + ROW_LOOKUP_COST + ROW_COPY_COST);
->
ROW_LOOKUP_COST= cost/rows - INDEX_BLOCK_COPY_COST - ROW_COPY_COST;
959.790773 / 1000000 - 3.56e-05 - 0.000060865547560
->
ROW_LOOKUP_COST= 0.0008633252254400001
From InnoDB RANGE SCAN we have ROW_LOOKUP_COST=0.000765972342436
From EQ_REF with index read we have KEY_LOOKUP_COST= 0.000791125387504,
which should in theory be identical to ROW_LOOKUP_COST,
For now we have to live with the difference (as I want to have the project done
for the next release).
The difference could be come from the following things:
- InnoDB estimation of rows in the range scan test is a bit off.
- Maybe the work to find a row from an internal key entry compared to
a external key is a bit difference (less checking/conversions)
- There is different keys used for range scan and this test that could have
different costs
- Maybe we should increase ROW_COPY_COST or ROW_LOOKUP_COST for InnoDB
and adjust other costs.
Some background. In range scan, the cost is:
- Scanning over all keys
- For each key, fetch row using rowid
For the EQ_REF cache
- Scan seq_1_to_1000000
for each value in seq
do a index_read() call
Archive scan cost
=================
table_scan time: 757.390280 ms
rows: 1000000
file size: 32260650 = 7878 IO_SIZE blocks
cost= scan_time() + TABLE_SCAN_SETUP_COST +
records * (ROW_COPY_COST + ROW_LOOKUP_COST + WHERE_COMPARE_COST);
757.390280 = scan_time() + 10 + 1000000 * (0.060866+0.032000)
->
scan_time()= 757.390280 - (10 + 1000000 * (0.060866+0.032000)/1000) = 654.52428
scan_time() is defined as:
cost.cpu= (blocks * DISK_READ_COST * DISK_READ_RATIO +
blocks * ARCHIVE_DECOMPRESS_TIME);
Default values for above:
blocks= 7878
DISK_READ_COST: 10.240000 usec
DIUSK_READ_RATIO= 0.20
->
ARCHIVE_COMPRESS_TIME= (654.52428 - (7878 * 10.240000/1000*0.2)) / 7878 =
0.081034543792841
MyRocksDB, TABLE SCAN
=====================
select sum(l_quantity) from check_costs_rocksdb;
table_scan 213.038648 ms
cost= blocks * avg_io_cost() *
optimizer_cache_cost * SCAN_LOOKUP_COST +
engine_blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST));
Some defaults:
optimizer_cache_cost = 0
index_block_copy_cost= 0.000035600 (Assume same as innoDB)
table_scan_setup_cost= 0 (Lets ignore it for now)
row_copy_cost=0.000060865547560 (Assume same as InnoDB for now)
show table status tells us that datalength=64699000 = 15795 4K-blocks.
cost= engine_blocks * INDEX_BLOCK_COPY_COST +
TABLE_SCAN_SETUP_COST +
rows * (ROW_NEXT_FIND_COST + ROW_COPY_COST))
ROW_NEXT_FIND_COST=
(costs - engine_blocks * INDEX_BLOCK_COPY_COST)/rows -
ROW_COPY_COST
= (213.03868 - 15796 * 0.000035600 - 0)/1000000 - 0.000060865547560 =
0.00015161079484
MyRocks INDEX SCAN
==================
select count(*) from test.check_costs_rocksdb force index (l_suppkey) where l_suppkey >= 0 and l_partkey >=0
index_scan 266.80435 ms
Note that myrocks returns 2M rows for the table when it has only 1M rows!
block_size= 8192
key_length= 18
compression=0.25 (75 %)
blocks= (key_length * rows) / 4 * block_size/4096 = 18 * 1000000/4 * 2=
2198 IO_BLOCKS (=1094 engine_blocks)
cost= keyread_time= blocks * avg_io_cost * DISK_READ_RATIO + engine_blocks * INDEX_BLOCK_COPY_COST + rows * (KEY_NEXT_FIND_COST + KEY_COPY_COST);
As we assume that everything is in memory (DISK_READ_RATIO=0)
->
cost= engine_blocks * INDEX_BLOCK_COPY_COST + rows * KEY_NEXT_FIND_COST;
Assuming INDEX_BLOCK_COPY_COST and KEY_COPY_COST are same as in Aria and InnoDB)
cost= 1094 * 3.56e-05 + 1000000 * (KEY_NEXT_FIND_COST + KEY_COPY_COST)
=
KEY_NEXT_FIND_COST + KEY_COPY_COST= (266.80435 - 1094 * 3.56e-05)/1000000
=
KEY_NEXT_FIND_COST= (266.80435 - 1094 * 3.56e-05)/1000000 - 0.000015685222496
->
KEY_NEXT_FIND_COST= 0.000251080181104
MyRocks EQ_REF with index_read
==============================
select straight_join count(*) from seq_1_to_1000000,test.check_costs_rocksdb where seq=l_linenumber
time: 857.548991
Here the engine first has to do a key lookup and copy the key to the upper
level (Index only read).
According to analyze statement:
- Cost for SELECT * from seq_1_to_1000000: 12.57 (See sequence_scan_cost)
- Time from check_costs: eq_ref_join: 857.548991
This is time for accessing both seq_1_to_1000000 and check_costs
time for check_cost_innodb: 857.548991-12.57 = 844.978991 ms
cost= rows * (keyread_time(1,1) + KEY_COPY_COST)
keyread_time(1,1)= INDEX_BLOCK_COPY_COST + ranges * KEY_LOOKUP_COST +
(rows-ranges) * KEY_NEXT_FIND_COST
As rows=1 and ranges=1:
keyread_time(1,1)= INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST
cost= rows * (KEY_COPY_COST + INDEX_BLOCK_COPY_COST + KEY_LOOKUP_COST)
->
KEY_LOOKUP_COST= cost/rows - INDEX_BLOCK_COPY_COST - KEY_COPY_COST;
844.978991 / 1000000 - 3.56e-05 - 0.000015685222496 = 0.000793693768504
MyRocks EQ_REF with clustered index read
========================================
select straight_join count(*) from seq_1_to_1000000,check_costs_rocksdb where seq=l_orderkey
eq_ref_cluster_join 1613.5670 ms
According to analyze statement:
- Cost for SELECT * from seq_1_to_1000000: 12.57 (See sequence_scan_cost)
- Time from check_costs: eq_ref_cluster_join: 1613.5670 ms
This is time for accessing both seq_1_to_1000000 and check_costs_innodb.
Time for check_cost_rocksdb: 1613.5670 - 12.57 = 1600.9970
cost= rows * (keyread_time(1,1) +
ranges * ROW_LOOKUP_COST +
(rows - ranges) * ROW_NEXT_FIND_COST +
rows * ROW_COPY_COST)
As rows=1 and ranges=1:
cost= rows * (INDEX_BLOCK_COPY_COST + ROW_LOOKUP_COST + ROW_COPY_COST);
->
ROW_LOOKUP_COST= cost/rows - INDEX_BLOCK_COPY_COST - ROW_COPY_COST;
1600.9970 / 1000000 - 3.56e-05 - 0.000060865547560 = 0.00150453145244
MyRocks Range scan
==================
select sum(l_orderkey) from test.check_costs_rocksdb force index(l_suppkey) where l_suppkey >= 0 and l_partkey >=0 and l_discount>=0.0
The MyRocks engine estimates the number of rows both for the table and
for the to be about 2M, double the real amount.
The timing and costs from check_costs.pl are:
range_scan time: 1845.06126 ms cost-where: 3698.8919 cost: 3730.8919
As the costs are about the double of the time, this is as good as we can do things until
MyRocks reported record count is corrected
The issue with wrongly estimated number of rows does not affect the other results from check_costs.pl
as table scans estimates uses the number of rows from the analyze, not from the engine.
Appendix
========
Future improvements
===================
The current costs are quite good for tables of 1M rows (usually about
10% from the true cost for the test table).
For smaller tables the costs will be a bit on the high side and for
bigger tables a bit on the low size for eq_ref joins (both with index
and with row lookup).
The only engine that takes into account the number of rows for key lookups
is heap with binary-tree indexes.
Ideas of how to fix this:
- Change KEY_LOOKUP_COST, INDEX_BLOCK_COPY_COST and ROW_LOOKUP_COST
(for clustered index) to take into account the height of the B tree.
Observations
============
Ratio between table scan and range scan
Queries used:
select sum(l_quantity) from check_costs_aria;
select sum(l_orderkey) from test.check_costs_aria force index(l_suppkey) where l_suppkey >= 0 and l_partkey >=0 and l_discount>=0.0;
The test for Aria shows that cost ratio of range_scan/table_scan are:
disk_read_ratio=0 341.745207/139.348286= 2.4524536097
disk_read_ratio=0.02 752.408528/145.748695= 5.1623688843
disk_read_ratio=0.20 4448.378423/203.352382= 21.8752216190
As we are using disk_read_ratio=0.02 by default, this means that in
mtr to not use table scan instead of range, we have to ensure that the
range does not cover more than 1/5 of the total rows.
Trying to understand KEY_COPY_COST
==================================
An index scan with 2 and 4 key parts on an Aria table.
The index has null key parts, so packed keys are used.
Query1 "index_scan" (2 integer key parts, both key parts may have NULLS):
select count(*) from $table force index (l_suppkey) where l_suppkey >= 0 and l_partkey >=0");
- Optimized build: Average 164 ms/query
- gprof build: Average 465 ms/query
[16] 51.2 0.00 0.21 3999987 handler::ha_index_next()
[15] 51.2 0.01 0.20 3999993 maria_rnext [15]
[22] 19.5 0.08 0.00 9658527 _ma_get_pack_key [22]
This means that for 3999987 read next calls, the time of _ma_get_pack_key
to retrieve the returned key is:
0.08 * (3999987/9658527)
The relation of KEY_COPY_COST to KEY_NEXT_FIND_COST is thus for Aria:
0.08 * (3999987/9658527)/0.21 = 0.15777 parts of KEY_NEXT_FIND_COST
------
Query 2 "index_scan_4_parts" (4 integer key parts, 2 parts may have NULL's):
select count(*) from $table force index (long_suppkey) where l_linenumber >= 0 and l_extra >0");
- Optimized build: 218 ms
- gprof build: Average 497 ms/query
Most costly functions
% cumulative self self total
time seconds seconds calls ms/call ms/call name
13.44 0.61 0.61 48292742 0.00 0.00 _ma_get_pack_key
8.59 1.00 0.39 28298101 0.00 0.00 ha_key_cmp
7.27 1.33 0.33 19999951 0.00 0.00 _ma_put_key_in_record
4.41 1.96 0.20 19999952 0.00 0.00 handler::ha_index_next(unsigned char*)
Call graph
[13] 9.0 0.20 0.21 19999952 handler::ha_index_next(unsigned char*) [13]
[3] 21.6 0.16 0.82 19999960 _ma_search_next [3]
[18] 7.7 0.02 0.33 19999951 _ma_read_key_record [18]
0.00 0.00 19887291/19999952 _ma_get_static_key [6565][19]
18.4 0.10 0.64 19999936 Item_cond_and::val_int() [19]
-> KEY_COPY_COST = 1.33/1.96 = 0.6785 parts of the index_read_next
Total cost increases from 2 -> 4 key parts = 1.96 / 1.40 = 40%
This includes the additional work in having more key pages, more work in
finding next key (if key parts are packed or possible null) ,and copying
the key parts to the record
I also did a quick analyze between using NOT NULL keys, in which case
Aria can use fixed key lengths. This gives a 39.4% speed up on index
scan, a small speedup to table scan (as 2 fields are cannot have null)
but not a notable speed up for anything else.
Trying to understand ROW_COPY_COST
==================================
An simple table scan on an Aria table
query: select sum(l_quantity) from check_costs_aria
From gprof running the above query 10 times with 1M rows in the table:
[14] 83.7 0.03 0.76 9999989 handler::ha_rnd_next()
[17] 51.6 0.49 0.00 10000010 _ma_read_block_record2 [17]
[18] 21.1 0.01 0.19 156359 pagecache_read [18]
The function that unpacks the row is _ma_read_block_record2()
Taking into account that all pages are cached:
(Note that the main cost in pagecache_read in this test is calculating the page
checksum)
ROW_COPY_COST/ROW_NEXT_FIND_COST= 0.49/(0.76+0.3-0.20) = 0.56977 = 0.57
Reason for SCAN_SETUP_COSTS
===========================
One problem with the new more exact cost model is that the optimizer
starts to use table scans much more for small tables (which is correct when
one looks at cost). However, small tables are usually cached fully so
it is still better to use index scan in many cases.
This problem is especially notable in mtr where most test cases uses
tables with very few rows.
TABLE_SCAN_SETUP_COST is used to add a constant startup cost for
table and index scans. It is by default set to 10 usec, about 10 MyISAM
row reads.
The following cost calculation shows why this is needed:
explain select count(*) from t1, t2 where t1.p = t2.i
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| 1 | SIMPLE | t1 | index | PRIMARY | PRIMARY | 4 | NULL | 2 | Using index |
| 1 | SIMPLE | t2 | ref | k1 | k1 | 5 | test.t1.p | 2 | Using index |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
t1 has 2 rows
t2 has 4 rows
Optimizer trace shows when using TABLE_SCAN_SETUP_COST=0:
index scan costs
"read_cost": 0.00308962,
read_and_compare_cost": 0.00321762
key read costs:
"rows": 2,
"cost": 0.00567934
CHOSEN:
Scan with join cache: cost": 0.0038774
rows_after_scan": 2
Note that in the following, we are using cost in microseconds while
the above costs are in milliseconds.
select * from information_schema.optimizer_costs where engine="myisam"\G
ENGINE: MyISAM
OPTIMIZER_DISK_READ_COST: 10.240000
OPTIMIZER_INDEX_BLOCK_COPY_COST: 0.035600
OPTIMIZER_KEY_COMPARE_COST: 0.008000
OPTIMIZER_KEY_COPY_COST: 0.066660
OPTIMIZER_KEY_LOOKUP_COST: 0.498540
OPTIMIZER_KEY_NEXT_FIND_COST: 0.060210
OPTIMIZER_DISK_READ_RATIO: 0.200000
OPTIMIZER_RND_POS_INTERFACE_COST: 0.000000
OPTIMIZER_ROW_COPY_COST: 0.088630
OPTIMIZER_ROW_LOOKUP_COST: 0.641150
OPTIMIZER_ROW_NEXT_FIND_COST: 0.049510
OPTIMIZER_ROWID_COMPARE_COST: 0.004000
@@OPTIMIZER_SCAN_SETUP_COST 10.000000
@@OPTIMIZER_WHERE_COST 0.032000
Checking the calculated costs:
index_scan_cost= 10.240000 * 0.2 + 0.035600 + 0.498540 + 4 * (0.060210+0.066660) = 3.08962
where_cost 0.032000*4= 0.128000
total: 3.21762
key_read_cost= 10.240000 * 0.2 + 0.035600 + 0.498540 + 0.060210 = 2.64235
key_copy_cost= 0.066660 * 2 = 0.13332
where_cost 0.032000*2= 0.06400
total: 2.64235 + 0.13332 + 0.06400 = 2.8396699999999999
Needs to be done 2 times (2 rows in t1): 5.67934
Join cache only needs 1 refill. The calculation is done in
sql_select.cc:best_access_path()
scan_with_join_cache=
scan_time + cached_combinations * ROW_COPY_COST * JOIN_CACHE_COST +
row_combinations * (ROW_COPY_COST * JOIN_CACHE_COST + WHERE_COST) =
3.2176 + 2 * 0.088630 + 2*2 * (0.088630 * 1 + 0.032000) =
3.87738
Other observations:
OPTIMIZER_KEY_NEXT_FIND_COST + OPTIMIZER_KEY_COPY_COST + OPTIMIZER_WHERE_COST=
0.060210 + 0.066660 + 0.032000 = 0.158870
OPTIMIZER_KEY_LOOKUP_COST / 0.158870 = 3.138
This means that when using index only reads (and DISK_READ_RATIO=0)
the optimizer will prefer to use 3 times more keys in range or ref
than doing a key lookups!
If DISK_READ_RATIO is higher, the above ratio increases. This is one of
the reasons why we set the default value for DISK_READ_RATIO quite low
(0.02 now)
(OPTIMIZER_ROW_COPY_COST + OPTIMIZER_ROW_NEXT_FIND_COST) /
(OPTIMIZER_KEY_COPY_COST + OPTIMIZER_KEY_NEXT_FIND_COST) =
(0.088630 + 0.049510) / (0.066660 + 0.060210) = 1.08831
Which means that table scans and index scans have almost the same cost.
select 0.066660
HEAP_TEMPTABLE_CREATE_COST
==========================
I added trackers in create_tmp_table() and open_tmp_table() and run a
simple query that create two materialized temporary table with an unique
index 31 times. I got the following tracking information:
(gdb) p open_tracker
$1 = {counter = 31, cycles = 302422}
(gdb) p create_tracker
$2 = {counter = 31, cycles = 1479836}
Cycles per create = (302422 + 1479836)/31= 57492
1000.0*57492/sys_timer_info.cycles.frequency = 0.0249 ms
HEAP_TMPTABLE_CREATE_COST= 0.025 ms
What to do with wrong row estimates
===================================
MyRocks can have a very bad estimate of rows, both for the number of rows in the table and also
for big ranges. Analyze table can fix this, but we have to consider how to keep the row estimate
correct when tables are growing over time.
Suggested fixed:
- If we can assume that the datafile size reported by the engine is somewhat correct, we could
estimate the number of rows as:
analyze_number_of_rows * current_datafile_size / analyze_datafile_size
MySQL cost structures
=====================
MySQL 8.0 server cost are stored in the class Server_cost_constants defined
int opt_costconstants.h
It containts the following slots and has the following default values:
m_row_evaluate_cost 0.1 Cost for evaluating the query condition on
a row
m_key_compare_cost 0.05 Cost for comparing two keys
m_memory_temptable_create_cost 1.0 Cost for creating an internal temporary
table in memory
m_memory_temptable_row_cost 0.1 Cost for retrieving or storing a row in an
internal temporary table stored in memory.
m_disk_temptable_create_cost 20.0 Cost for creating an internal temporary
table in a disk resident storage engine.
m_disk_temptable_row_cost 0.5 Cost for retrieving or storing a row in an
internal disk resident temporary table.
Engine cost variables:
m_memory_block_read_cost 0.25 The cost of reading a block from a main
memory buffer pool
m_io_block_read_cost 1.0 The cost of reading a block from an
IO device (disk)
-------
Some cost functions:
scan_time() = data_file_length / IO_SIZE + 2;
read_time(index, ranges, rows)= rows2double(ranges + rows);
index_only_read_time()= records / keys_per_block
table_scan_cost()= scan_time() * page_read_cost(1.0);
index_scan_cost()= index_only_read_time(index, rows) *
page_read_cost_index(index, 1.0);
read_cost()= read_time() * page_read_cost(1.0);
page_read_cost()= buffer_block_read_cost(pages_in_mem) +
io_block_read_cost(pages_on_disk);
io_block_read_cost()= blocks * m_io_block_read_cost
buffer_block_read_cost()= blocks * m_memory_block_read_cost;
There are also:
table_in_memory_estimate()
index_in_memory_estimate()
If the storage engine is not providing estimates for the above, then
the estimates are done based on table size (not depending on how many
rows are going to be accessed in the table).
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