Dealing with missing metrices in calculations

Question

I use a formula to calculate approx. maximum memory consumption of MySQL instances that (simplified and written in PromQL) reads:

(
    mysql_global_variables_key_buffer_size +
    mysql_global_variables_query_cache_size +
    mysql_global_variables_tmp_table_size +
    mysql_global_variables_innodb_buffer_pool_size +
    mysql_global_variables_innodb_additional_mem_pool_size +
    mysql_global_variables_innodb_log_buffer_size +
    (
        mysql_global_variables_max_connections *
        (
            mysql_global_variables_sort_buffer_size +
            mysql_global_variables_read_buffer_size +
            mysql_global_variables_read_rnd_buffer_size +
            mysql_global_variables_join_buffer_size +
            mysql_global_variables_thread_stack +
            mysql_global_variables_binlog_cache_size
        )
    )
)

Unfortunately the mysql_global_variables_innodb_additional_mem_pool_size metric is not always present for every instance resulting in "no data" if this it is included in the calculation.

There's the absent(v instant-vector) function that may be used to solve this but I am not sure how.

I would like nonexistent metrices be replaced with a constant (0 in this case). Is it possible?

Could you please provide me with some hints on how to deal with missing metrices in calculations in PromQL?

Answer 1

mysql_global_variables_innodb_additional_mem_pool_size or up * 0

https://www.robustperception.io/existential-issues-with-metrics/ looks at this problem in more detail.

Answer 2

Accepted solution won't work well in case we would try to sum two metrics both of them can be missing. In my specific case it is mysql_info_schema_innodb_metrics_transaction_trx_rseg_history_len (from mariadb) and mysql_global_status_innodb_history_list_length (from mysql). Provided solution gave me 3 graphs for a single host.

I used the following workaround:

(metric1{hostname="h"} or on() vector(0))+(metric2{hostname="h"} or on() vector(0))

taken from here: https://github.com/grafana/grafana/issues/2393#issuecomment-192522042