Mariadb – How to speed up this query

indexmariadbmariadb-10.1performancequery-performance

Here is the definition of the relevant table:

CREATE TABLE IF NOT EXISTS pricehistory (
        id INTEGER(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
        price INTEGER NOT NULL,
        region VARCHAR(20) NOT NULL,
        timestamp TIMESTAMP NOT NULL,
        is_from_api INTEGER DEFAULT 0)

Here is the query at issue:

SELECT MAX(price) AS price 
FROM pricehistory 
WHERE UNIX_TIMESTAMP(CONVERT_TZ(timestamp, '+00:00', 'SYSTEM')) 
      > 
      UNIX_TIMESTAMP(CONVERT_TZ(DATE_SUB(NOW(), INTERVAL 1000 DAY), '+00:00', 'SYSTEM')) 
  AND region='us';

Here is the Explain

id: 1
select_type: SIMPLE
table: pricehistory
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 368040
Extra: Using where

Ok so this query runs really slow. I think it takes about 500ms to run for only 300k-400k rows.

Roughly 1/5 of all the rows have a region='us'. If I remove the UNIX_TIMESTAMP() function it drops the run time by about 100ms so I'll probably do that.

I've tried indexing, but I'm new to indexing and have never done it before. All the indexing I've done has either had no effect or slowed the query down even more.

I need the max price in the last x days, of only 1/5 of the rows (region='us') in the whole table. I can't think of a way to build an index out of that.

Note: I also have a query that gets the minimum price.

Server version: 10.1.35-MariaDB – mariadb.org binary distribution.

Best Answer

Because your WHERE clasue referencing timestamp has the column wrapped in a function, it is not going to be able to use an index efficiently to may currently be scanning the whole table.

An index on region may help, but not a lot unless that column is quite selective (or at all if the majority of the rows are from the region you are looking at).

As you are applying the same functions to the column and the value you are searching through it with, you can just drop that: the inequality

UNIX_TIMESTAMP(CONVERT_TZ(timestamp, '+00:00', 'SYSTEM')) > UNIX_TIMESTAMP(CONVERT_TZ(DATE_SUB(NOW(), INTERVAL 1000 DAY), '+00:00', 'SYSTEM'))

is exactly equivelant to just

timestamp > DATE_SUB(NOW(), INTERVAL 1000 DAY)

and that will allow in index on timestamp to be useful.

With that change in place, a combined index on both timestamp and region is likely to be most helpful here. Perhaps also adding price to the end then the query running will not need separate lookups to find that column after filtering by the others (though this will increase the size of the index so you are trading space for perhaps only a small amount of speed).

Related Solutions

Mysql – Identical query, tables, but different EXPLAIN and performance

Just make a simple join. Sub-queries does not provide the best result quite often

EXPLAIN SELECT l.id, l.level_name, l.date_published, l.rating
FROM levels AS l
INNER JOIN users_favorites AS uf 
ON uf.level_id = l.id
WHERE l.user_id = 2;

MySQL – How to Decide Which Execution Plan is Better

Multiplying the rows is invalid for several reasons:

Many times, the rows examined are an approximation (based on statistics, not accurate), good for query plan selection, but not for performance calculation
The total number of rows examined on a nested loop join (A, B) is not rows_examined_on_table_A * rows_examined_on_table_B, but rows_examined_on_table_A + rows_returned_from_table_A * rows_examined_on_table_B. Where clauses can make a huge difference on that, although it is true that the mentioned calculations is many times used as a broad approximation, assuming the indexes are being created properly and the main causes of filtering out results.
Modern MySQL versions do not use always a nested loop join approach for executing joins and subqueries. Check 5.6 subquery optimizations and other optimization documents on the same manual. Additionally, some of the new optimization techniques do not modify the predicted examined rows, which at some times can be way lower than the one printed, even if it has been calculated exactly.

In particular, on your first query, you are hitting a well know MySQL bug? limitation? in which an IN subquery is identified as a DEPENDENT SUBQUERY, even if it really isn't, forcing the outmost query to be executed without an index (full table scan) in order to test all possible values of the first table. That is usually an indicator that it is a bad query. It seems not to bee too bad in this case, as the table is small, but it is usually an indication of bad performance.

The other thing that should bring your attention is the Using temporary; Using filesort. Filtering is not the only thing where you should focus, as these extra pieces of information are telling you that a large sorting has to be done using a temporary table (that may or may not end up on disk, but at least has to be materialized). That is another indicator of potential bad performance, that in some cases can be avoided with the right indexes.

I will not tell you which is the right query to use (partially, because I do not know all the variables: indexes, tables structure, etc., and in most cases it will depend on the particular hardware/resources available), but I will tell you the tools to decide:

Profile the query- obtain the post execution times and how much of it it is being invested in what. You can use SHOW PROFILES up to 5.5, and the performance_schema starting with 5.6.
As time can be sometimes variable (for example, depending on other queries being executed at the same time, depending on the buffer pool contents) Obtain post-execution statistics with SHOW SESSION STATUS. In particular:
```
FLUSH SESSION STATUS;
SELECT ... ;
SHOW STATUS like 'Hand%';
```
will give you the exact number of handler calls done (approximately, the number of rows read and written for that particular query- although that is not 100% accurate, as it depends on the particular engine implementation).

You may also want to monitor other status variables, like the created temporary tables, created temporary tables on disk and sort passes/sorted rows.

All of these will give you post-execution, exact, time-independent parameters to evaluate the performance of a query. Percona even has a patch for the slow log to output that information on the logs instead of using performance_schema.

With those extra pieces of information you will be able to evaluate more objectively which query is better, and not relying exclusively on EXPLAIN, which only provides limited pre-execution information.

Best Answer

Related Solutions

Mysql – Identical query, tables, but different EXPLAIN and performance

MySQL – How to Decide Which Execution Plan is Better

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