原文地址：http://MysqL.rjweb.org/doc.PHP/ricksrots

Brought to you by Rick James

Here are 160+ tips,tricks,suggestions,etc. They come from a decade of improving performance in MysqL in thousands of situations. There are exceptions to the statements below,but they should help guide you into better understanding how to effectively use MysqL.

SELECTs -- do's and don'ts

RoTs dt >= '2010-02-01' AND dt < '2010-02-01' + INTERVAL 7 DAY ⚈ ORDER BY NULL -- a little-known trick to avoid GROUP BY doing a sort (if there is another way). ⚈ WHERE (a,b) > (7,8) is poorly optimized ⚈ Gather these to study a slow query: SHOW CREATE TABLE,SHOW TABLE STATUS,EXPLAIN. ⚈ Do not use OFFSET for pagination -- continue where you "left off" ⚈ Don't mix DISTINCT and GROUP BY ⚈ Be explicit about UNION ALL vs UNION DISTINCT -- it makes you think about which to use ⚈ Do not use SELECT * except for debugging or when fetching into a hash. ⚈ VIEWs are poorly optimized ⚈ A subquery in the FROM clause may be useful for retrieving BLOBs without sorting them: Speed up a query by first finding the IDs,then self-JOIN to fetch the rest. MysqL rather late in the game. They have not been well optimized,so it is usually better to turn your SELECTs into an equivalent JOIN. This is especially true for "IN ( SELECT ... )",but that is better optimized in 5.6.5 and MariaDB 5.5. Sometimes a subquery is really the best way to optimize a SELECT. The common thread of these "good" subqueries seems to be when the subquery has to scan a lot of rows,but boils down the intermediate resultset to a small number of rows. This is likely to happen with GROUP BY or LIMIT in the subquery. Index hints (FORCE INDEX,etc) may help you today,but may be the wrong thing for tomorrow -- different constants in the WHERE clause may lead FORCE to do the "wrong" thing. For analyzing a slow query,SHOW CREATE TABLE provides the datatypes,indexes,and engine. (DESCRIBE provides much less info.) SHOW TABLE STATUS tells how big the table is. EXPLAIN says how the query optimizer decided to perform the query. It is so tempting to use ORDER BY id LIMIT 30,10to find the 4th page of 10 items. But it is so inefficient,especially when you have thousands of pages. The thousandth page has to read (at some level) all the pages before it. "Left off" refers to having the "Next" button on one page give the id (or other sequencing info) of where the next page can be found. Then that page simply does WHERE id > $leftoff ORDER BY id LIMIT 10. </tr>

INDEXing

Discussion

RoTs MysqL rarely uses two INDEXes in one SELECT. Main exceptions: subqueries,UNION. ⚈ A "prefix" index -- INDEX(name(10)) -- is rarely useful. Exception: TEXT ⚈ A UNIQUE "prefix" is probably wrong -- UNIQUE(name(10)) forces 10 chars to be unique. ⚈ It is ok to have Index_length > Data_length ⚈ 5 fields in a compound index seems "too many" ⚈ Having no compound indexes is a clue that you do not understand their power. INDEX(a,b) may be much better than INDEX(a),INDEX(b) ⚈ INDEX(a,b) covers for INDEX(a),so drop the latter. ⚈ 2x speedup when "Using index" (a "covering" index) ⚈ Akiban (3rd party) "groups" tables together,interleaved,to improve JOIN performance. ⚈ FULLTEXT (MyISAM) -- watch out for ft_min_word_len=4,stopwords,and 50% rule ⚈ A FULLTEXT index will be used before any other index. ⚈ FULLTEXT -- consider Syphinx,Lucene,etc (3rd Party) all the fields needed in a SELECT are included in the INDEX.</tr>

ENGINE Differences

Discussion

RoTs </tr>

Optimizations,and not

Discussion

RoTs If you can arrange for rows to be "adjacent" to each other,then one disk fetch will bring in many rows (10x speedup). "Batched" INSERTs are where one INSERT statement has multiple rows. Nearly all of the performance benefit is in the first 100 rows; going beyond 1000 is really getting into 'diminishing returns'. Furthermore,in a Replication environment,a huge INSERT would cause the Slave to get 'behind'. </tr>

PARTITIONing

Discussion

RoTs 1M rows ⚈ No more than 50 PARTITIONs on a table (open,show table status,are impacted) (fixed in 5.6.6?) ⚈ PARTITION BY RANGE is the only useful method. ⚈ SUBPARTITIONs are not useful. ⚈ The partition field should not be the field first in any key. ⚈ It is OK to have an AUTO_INCREMENT as the first part of a compound key,or in a non-UNIQUE index. opening the table.) It does not have to be the only field,nor does it have to be PRIMARY or UNIQUE. If it is not UNIQUE,you could INSERT a duplicate id if you explicitly provide the number. </tr>

Memory Usage

Discussion

RoTs 1/sec,increase table_open_cache. ⚈ Turn off the Query Cache. Type=off and size=0 MysqL performance depends on being in control of its use of RAM. The biggest pieces are the caches for MyISAM or InnoDB. These caches should be tuned to use a large chunk of RAM. Other things that can be tuned rarely matter much,and the default values in my.cnf (my.ini) tend to be "good enough". The "Query Cache" is totally distinct from the key_buffer and the buffer_pool. ALL QC entries for one table are purged when ANY change to that table occurs. Hence,if a table is being frequently modified,the QC is virtually useless. </tr>

Character Sets

Discussion

RoTs utf8_general_ci > utf8_bin ⚈ Debug stored data via HEX(col),LENGTH(col),CHAR_LENGTH(col) ⚈ Do not use utf8 for hex or ascii strings (GUID,md5,ip address,country code,postal code,etc.) = CHAR_LENGTH(col): with European text '=' for latin1,'>' for utf8. </tr>

Datatypes - Directly supported

Discussion

RoTs WHERE a.start < b.end AND a.end > b.start ⚈ Don't be surprised by AUTO_INCREMENT values after uncommon actions. More cacheable --> Faster. An AUTO_INCREMENT is very non-random,at least for inserting. Each new row will be on the 'end' of the table. That is,the last block is "hot spot". Thanks to caching very little I/O is needed for an AUTO_INCREMENT index. VARCHAR(255) for everything is tempting. And for "small" tables it won't hurt. For large tables one needs to consider what happens during the execution of complex SELECTs. If a "temporary" table is implicitedly generated,the VARCHAR will take 767 bytes in the temp table (2+3*255) bytes. 2=VAR overhead,3=utf8 expansion,255=your limit. A DELETE of the last row may or many not burn that AUTO_INCREMENT id. INSERT IGNORE burns ids because it allocates values before checking for duplicate keys. A Slave may see InnoDB ids arriving out of order (because transactions arrive in COMMIT order). A ROLLBACK (explicit or implicit) will burn any ids already allocated to INSERTs. REPLACE = DELETE + INSERT,so the INSERT comments apply to REPLACE. After a crash,the next id to be assigned may or may not be what you expect; this varies with Engine. </tr>

Datatypes - Implicit

Discussion

RoTs Since GUID,UUID,MD5,and SHA1 are fixed length,VAR is not needed. If they are in hex,don't bother with utf8; use BINARY or CHAR CHARSET ascii. Images could be stored in BLOB (not TEXT). This better assures referential integrity (not accidentally deleting the Metadata or image,but not both). On the other hand,it is clumsy. With files,an img tag can point directly to the image on disk. </tr>

Hardware

Discussion

RoTs 8 cores degrade performance. (Changes coming in XtraDB,5.6,MariaDB,etc) (5.6 claims to be good to 48 cores - YMMV; 5.7 claims 64) ⚈ A single connection will not use more than one core. Not even with UNION or PARTITION. ⚈ Don't put a cache in front of a cache ⚈ 10x speed up when disk blocks are cached,so... Time a query twice -- first will get things cached,second will do no I/O ⚈ Benchmark with "SELECT SQL_NO_CACHE ..." (to avoid Query cache) </tr>

PXC / Galera

Discussion

RoTs or slower than traditional replication ⚈ AUTO_INCREMENT values won't be consecutive ⚈ Handle "critical reads" using wsrep_causal_reads ⚈ ALTERs need to be handled differently (see RSU vs TOI) ⚈ Lots of tricks are based on: remove from cluster + do stuff + add back to cluster ⚈ Minimal HA: 1 node in each of 3 datacenters; one could be just a grabd </tr>

Data Warehouse

Discussion

RoTs </tr>

Miscellany

Discussion

RoTs MysqL can run 1000 qps. (just a RoT; YMMV) ⚈ The SlowLog is the main clue into performance problems. Keep it on. Use long_query_time=2. ⚈ 1000+ Databases or tables is a clue of poor schema design ⚈ 10,000+ Databases or tables will run very slowly because of OS overhead ⚈ < 10% improvement -- don't bother. Exception: shrink the datatypes before deploying ⚈ Beware of SQL injection ⚈ If you can't finish an InnoDB transaction in 5 seconds,redesign it. ⚈ MySQL has many builtin 'hard' limits; you will not hit any of them. ⚈ An excessive MaxClients (Apache) can cause trouble with max_connections ⚈ Connection pooling is generally not worth the effort. (Not to be confused with 5.7's Thread Pooling.) ⚈ SBR vs RBR -- too many variables to make a call ⚈ A Slave can have only one Master. (Exceptions: 5.7,~Galera) ⚈ Do all ALTERs in a single statement. Exceptions: PARTITION,NDB,some cases in 5.6+ ⚈ ALTER to add an ENUM option is efficient. (This has not always been the case.) ⚈ "Load average" often raises false alarms. ⚈ Pick carefully between REPLACE (== DELETE + INSERT),INSERT IGNORE,and INSERT ... ON DUPLICATE KEY UPDATE. ⚈ When Threads_running > 10,you may be in serIoUs trouble. ⚈ SHOW PROCESSLIST with some threads "Locked" -- some other thread is hogging something. ⚈ SHOW PROCESSLIST may fail to show the locking thread -- it is Sleeping,but not yet COMMITted. ⚈ >90% cpu --> investigate queries/indexes. (The SlowLog also catches such.) ⚈ >90% of one core -- since MysqL won't use multiple cores in a single connection,this indicates an inefficient query. (Eg,12% overall on an 8-core Box is probably consuming one core.) ⚈ >90% I/O -- tuning,overall schema design,missing index,etc. ⚈ "Nosql" is a catchy phrase looking for a definition. By the time Nosql gets a definition,it will look a lot like an RDBMS solution. MysqL can run thousands of trivial queries on modern hardware. Some special benchmarks have driven InnoDB past 100K qps. At the other extreme,I have seen a query run for a month. 1000 qps is simply a RoT that applies to a lot of systems; but your mileage can really vary a lot. Over-normalization can lead to inefficiencies. Why have a 4-byte INT as an id for the 200 countries in the world; simply use a 2-byte CHAR(2) CHARSET ascii. Don't normalize dates -- see Data Warehousing. sql Injection is where you take user input (say,from an HTML form) and insert it verbatim into a sql statement. Some hacker will soon find that your site is not protecting itself and have his way with your data. "SELECT *" will break your code tomorrow when you add another field. It is better to spell out the fields explicitly. (There is no noticeable performance difference.) ALTER,in most situations,completely copies over the table and rebuilds all the indexes. For a huge table,the can take days. Doing two ALTERs means twice the work; A single ALTER statement with several operations in it. OPTIMIZE is similarly costly,and may not provide much benefit. MariaDB 5.3's "Dynamic Columns" eats into a big excuse for "NoSQL". </tr>

原文链接：https://www.f2er.com/mariadb/73678.html