Limit max prepared statement count - mysql

Question

The problem
I wrote an application which synchronizes data from BigQuery into a MySQL database. I try to insert roughly 10-20k rows in batches (up to 10 items each batch) every 3 hours. For some reason I receive the following error when it tries to upsert these rows into MySQL:
Can't create more than max_prepared_stmt_count statements:
Error 1461: Can't create more than max_prepared_stmt_count statements
(current value: 2000)
My "relevant code"
// ProcessProjectSkuCost receives the given sku cost entries and sends them in batches to upsertProjectSkuCosts()
func ProcessProjectSkuCost(done <-chan bigquery.SkuCost) {
var skuCosts []bigquery.SkuCost
var rowsAffected int64
for skuCostRow := range done {
skuCosts = append(skuCosts, skuCostRow)
if len(skuCosts) == 10 {
rowsAffected += upsertProjectSkuCosts(skuCosts)
skuCosts = []bigquery.SkuCost{}
}
}
if len(skuCosts) > 0 {
rowsAffected += upsertProjectSkuCosts(skuCosts)
}
log.Infof("Completed upserting project sku costs. Affected rows: '%d'", rowsAffected)
}
// upsertProjectSkuCosts inserts or updates ProjectSkuCosts into SQL in batches
func upsertProjectSkuCosts(skuCosts []bigquery.SkuCost) int64 {
// properties are table fields
tableFields := []string{"project_name", "sku_id", "sku_description", "usage_start_time", "usage_end_time",
"cost", "currency", "usage_amount", "usage_unit", "usage_amount_in_pricing_units", "usage_pricing_unit",
"invoice_month"}
tableFieldString := fmt.Sprintf("(%s)", strings.Join(tableFields, ","))
// placeholderstring for all to be inserted values
placeholderString := createPlaceholderString(tableFields)
valuePlaceholderString := ""
values := []interface{}{}
for _, row := range skuCosts {
valuePlaceholderString += fmt.Sprintf("(%s),", placeholderString)
values = append(values, row.ProjectName, row.SkuID, row.SkuDescription, row.UsageStartTime,
row.UsageEndTime, row.Cost, row.Currency, row.UsageAmount, row.UsageUnit,
row.UsageAmountInPricingUnits, row.UsagePricingUnit, row.InvoiceMonth)
}
valuePlaceholderString = strings.TrimSuffix(valuePlaceholderString, ",")
// put together SQL string
sqlString := fmt.Sprintf(`INSERT INTO
project_sku_cost %s VALUES %s ON DUPLICATE KEY UPDATE invoice_month=invoice_month`, tableFieldString, valuePlaceholderString)
sqlString = strings.TrimSpace(sqlString)
stmt, err := db.Prepare(sqlString)
if err != nil {
log.Warn("Error while preparing SQL statement to upsert project sku costs. ", err)
return 0
}
// execute query
res, err := stmt.Exec(values...)
if err != nil {
log.Warn("Error while executing statement to upsert project sku costs. ", err)
return 0
}
rowsAffected, err := res.RowsAffected()
if err != nil {
log.Warn("Error while trying to access affected rows ", err)
return 0
}
return rowsAffected
}
// createPlaceholderString creates a string which will be used for prepare statement (output looks like "(?,?,?)")
func createPlaceholderString(tableFields []string) string {
placeHolderString := ""
for range tableFields {
placeHolderString += "?,"
}
placeHolderString = strings.TrimSuffix(placeHolderString, ",")
return placeHolderString
}
My question:
Why do I hit the max_prepared_stmt_count when I immediately execute the prepared statement (see function upsertProjectSkuCosts)?
I could only imagine it's some sort of concurrency which creates tons of prepared statements in the meantime between preparing and executing all these statements. On the other hand I don't understand why there would be so much concurrency as the channel in the ProcessProjectSkuCost is a buffered channel with a size of 20.

Answer 1

You need to close the statement inside upsertProjectSkuCosts() (or re-use it - see the end of this post).
When you call db.Prepare(), a connection is taken from the internal connection pool (or a new connection is created, if there aren't any free connections). The statement is then prepared on that connection (if that connection isn't free when stmt.Exec() is called, the statement is then also prepared on another connection).
So this creates a statement inside your database for that connection. This statement will not magically disappear - having multiple prepared statements in a connection is perfectly valid. Golang could see that stmt goes out of scope, see it requires some sort of cleanup and then do that cleanup, but Golang doesn't (just like it doesn't close files for you and things like that). So you'll need to do that yourself using stmt.Close(). When you call stmt.Close(), the driver will send a command to the database server, telling it the statement is no longer needed.
The easiest way to do this is by adding defer stmt.Close() after the err check following db.Prepare().
What you can also do, is prepare the statement once and make that available for upsertProjectSkuCosts (either by passing the stmt into upsertProjectSkuCosts or by making upsertProjectSkuCosts a func of a struct, so the struct can have a property for the stmt). If you do this, you should not call stmt.Close() - because you aren't creating new statements anymore, you are re-using an existing statement.
Also see Should we also close DB's .Prepare() in Golang? and https://groups.google.com/forum/#!topic/golang-nuts/ISh22XXze-s