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Query information_schema compatible with MySQL 8 #2652

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Jan 13, 2022
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Query information_schema compatible with MySQL 8 #2652

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741258f
Query information_schema compatible with MySQL 8
px3303 Jan 11, 2022
1cad5fa
Optimize and normalize code
px3303 Jan 13, 2022
14a02e7
Update CHANGELOG file
px3303 Jan 13, 2022
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32 changes: 32 additions & 0 deletions quota/mysqlqm/mysql_quota.go
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,8 @@ import (
"context"
"database/sql"
"errors"
"fmt"
"strconv"

"github.com/google/trillian/quota"
)
Expand Down Expand Up @@ -97,6 +99,10 @@ func (m *QuotaManager) countUnsequenced(ctx context.Context) (int, error) {
}

func countFromInformationSchema(ctx context.Context, db *sql.DB) (int, error) {
// turn off statistics caching for MySQL 8
if err := turnOffInformationSchemaCache(ctx, db); err != nil {
return 0, err
}
// information_schema.tables doesn't have an explicit PK, so let's play it safe and ensure
// the cursor returns a single row.
rows, err := db.QueryContext(ctx, countFromInformationSchemaQuery, "Unsequenced", "BASE TABLE")
Expand Down Expand Up @@ -124,3 +130,29 @@ func countFromTable(ctx context.Context, db *sql.DB) (int, error) {
}
return count, nil
}

// 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.

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 { ...?

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ok

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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.

if err != nil {
if errors.Is(err, sql.ErrNoRows) {
return nil
}

return fmt.Errorf("Failed to get variable %q: %v", opt, err)
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nit: "Failed" -> "failed"

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ok

}

exp, err := strconv.Atoi(expiry)
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Can inline here too?
if exp, err := strconv...; err != nil || exp != 0 { ...

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removed

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)
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nit: "Failed" -> "failed"

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ok

}
}

return nil
}