Query information_schema compatible with MySQL 8 #2652
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Turn off statistics caching for MySQL 8. To always retrieve the latest statistics directly from the storage engine and bypass cached values, set information_schema_stats_expiry to 0. See https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_information_schema_stats_expiry
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Hi px3303, many thanks for the PR! To help understanding the context for it, would you mind updating the description with a bit of background on why this change is necessary/what [potential] problem it fixes? Thanks in advance. |
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Ah, that's great - thank you! |
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/gcbrun |
quota/mysqlqm/mysql_quota.go
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| return nil | ||
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| return fmt.Errorf("Failed to get variable %q: %v", opt, err) |
quota/mysqlqm/mysql_quota.go
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| exp, err := strconv.Atoi(expiry) | ||
| if err != nil || exp != 0 { | ||
| if _, err := db.ExecContext(ctx, "SET SESSION "+opt+"=0"); err != nil { | ||
| return fmt.Errorf("Failed to set variable %q: %v", opt, err) |
quota/mysqlqm/mysql_quota.go
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| opt := "information_schema_stats_expiry" | ||
| res := db.QueryRowContext(ctx, "SHOW VARIABLES LIKE '%"+opt+"%'") | ||
| var none, expiry string | ||
| err := res.Scan(&none, &expiry) |
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nit: Can you inline this with the check on err like this: if err := res.Scan...; err != nil { ...?
quota/mysqlqm/mysql_quota.go
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| return fmt.Errorf("Failed to get variable %q: %v", opt, err) | ||
| } | ||
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| exp, err := strconv.Atoi(expiry) |
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Can inline here too?
if exp, err := strconv...; err != nil || exp != 0 { ...
| // turnOffInformationSchemaCache turn off statistics caching for MySQL 8 | ||
| // To always retrieve the latest statistics directly from the storage engine and bypass cached values, set information_schema_stats_expiry to 0. | ||
| // See https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_information_schema_stats_expiry | ||
| func turnOffInformationSchemaCache(ctx context.Context, db *sql.DB) error { |
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Will this fail safely for all versions of MySQL prior to 8 or should we check the version and only execute on version 8+?
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Yes, it will fail safely for versions before MySQL 8.0. This is because this function uses SHOW VARIABLES LIKE statement. Executing this statement in versions before MySQL 8 is equivalent to searching a system variable that does not exist, and it will only return empty result rather than error.
Furthermore, tests of 5.6.51, 5.7.36 and 8.0.27 have already passed locally.
quota/mysqlqm/mysql_quota.go
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| opt := "information_schema_stats_expiry" | ||
| res := db.QueryRowContext(ctx, "SHOW VARIABLES LIKE '%"+opt+"%'") | ||
| var none, expiry string | ||
| err := res.Scan(&none, &expiry) |
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Is it possible to Scan expiry directly into an int or will that fail if it's unset?
(Would changing the query to SELECT FROM @@SESSION.information_schema_stats_expiry help if so?)
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Thanks very much for your reminding. Indeed, “expiry” should be scanned into int type.
When this variable does not exist (versions before MySQL 8.0), SELECT @@SESSION.information_schema_stats_expiry statement will return error instead of empty resultset.
Standardize error messages and code style. More rigorous SQL statements.
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I have updated the |
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/gcbrun |
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@px3303 - Nice one, many thanks! |
Turn off statistics caching for MySQL 8.
To always retrieve the latest statistics directly from the storage engine and bypass cached values, set information_schema_stats_expiry to 0.
See https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_information_schema_stats_expiry
Fixes #2653
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