3.3 KiB
3.3 KiB
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SQL tuning
Introduction
SQL tuning is a broad topic and many books have been written as reference.
It's important to benchmark and profile to simulate and uncover bottlenecks.
- Benchmark - Simulate high-load situations with tools such as ab .
- Profile - Enable tools such as the slow query log to help track performance issues.
Benchmarking and profiling might point you to the following optimizations.
Tighten up the schema
- MySQL dumps to disk in contiguous blocks for fast access.
- Use
CHAR
instead ofVARCHAR
for fixed-length fields.CHAR
effectively allows for fast, random access, whereas withVARCHAR
, you must find the end of a string before moving onto the next one.
- Use
TEXT
for large blocks of text such as blog posts.TEXT
also allows for boolean searches. Using aTEXT
field results in storing a pointer on disk that is used to locate the text block. - Use
INT
for larger numbers up to 2^32 or 4 billion. - Use
DECIMAL
for currency to avoid floating point representation errors. - Avoid storing large
BLOBS
, store the location of where to get the object instead. VARCHAR(255)
is the largest number of characters that can be counted in an 8 bit number, often maximizing the use of a byte in some RDBMS.- Set the
NOT NULL
constraint where applicable to improve search performance .
Use good indices
- Columns that you are querying (
SELECT
,GROUP BY
,ORDER BY
,JOIN
) could be faster with indices. - Indices are usually represented as self-balancing B-tree that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time.
- Placing an index can keep the data in memory, requiring more space.
- Writes could also be slower since the index also needs to be updated.
- When loading large amounts of data, it might be faster to disable indices, load the data, then rebuild the indices.
Avoid expensive joins
- Denormalize where performance demands it.
Partition tables
- Break up a table by putting hot spots in a separate table to help keep it in memory.
Tune the query cache
- In some cases, the query cache could lead to performance issues .