same query gets slower after other queries get called (using a lot of prepared statements inside stored procedures)

Question

I have a query that, when i run it first after i restart mysql, takes 2 seconds, but when I run a sequence of other queries before it (this query belongs to a procedure between other procedures) suddenly the query takes around 2 minutes, If i restart mysql and rerun it it takes again 2 seconds.

The durations are almost the same (~2 seconds and ~ 2 minutes) so even when it slows down it's not random, and it's specifically this query that gets slower (or maybe this table), everything else is normal.

• Disabled query_cache
• increased innodb_buffer_pool_size (I assumed it had to do with a memory or a buffer getting full making the next query slower)
• checked lock tables

I don't know what else (except the data) can a query affect another query/table after it.

Is there any path to search for or something to try, I don't have any lead on what to search for.

---

I'm using MySQL version 5.7

the explain plan is

I actually found out that the issue is not with a specific table, but simply too many prepared statements

+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| Com_prepare_sql    | 60    |
| Com_stmt_prepare   | 82    |
| Com_xa_prepare     | 0     |
| Com_stmt_reprepare | 24    |
+--------------------+-------+

It is mentionned in the doc that A prepared statement is also global to the session. If you create a prepared statement within a stored routine, it is not deallocated when the stored routine ends.

I thought this meant that they should be deallocated explicitely, but apparently this is not the case.

insert into s__demand_table_14_0(year, hospital_id, disease_id, nb_patients)
        SELECT fcbf_parent.year,
           table_referral.to_hospital_id,
           table_referral.disease_id,
           SUM(ifnull(fcbf_parent.nb_patients, 0) *
               ifnull(table_referral.ratio_of_referrals * (1 + ifnull(ir.ratio_of_referrals, 0)), 0)) +
           ifnull(SUM(fcbf.nb_patients), 0) AS referred_patients
    FROM table_hospital bf_current
             join table_hospital bf_parent on bf_parent.type = ? or bf_parent.type = ?
             LEFT JOIN
         table_referral ON bf_current.type = ? and table_referral.to_hospital_id = bf_current.id and table_referral.from_hospital_id = bf_parent.id
            JOIN s__demand_catchment_by_hospital_14_0 fcbf_parent on
            fcbf_parent.disease_id = table_referral.disease_id
            and fcbf_parent.hospital_id = bf_parent.id
            and fcbf_parent.nb_patients > 0
             LEFT JOIN s__demand_catchment_by_hospital_14_0 fcbf on
            fcbf.disease_id = table_referral.disease_id
            and fcbf.hospital_id = bf_current.id
             left join s__intermediate_referral_14_0 ir
                       on (ir.from_hospital_type = ? OR ir.from_hospital_type = ?) and
                          ir.to_hospital_type = ? and ir.year = fcbf_parent.year and ir.disease_id = table_referral.disease_id
    where bf_current.type = ?
    GROUP BY fcbf_parent.year, table_referral.to_hospital_id, table_referral.disease_id;

SHOW GLOBAL STATUS LIKE 'com_%_sql';

| Variable_name   | Value |
+-----------------+-------+
| Com_dealloc_sql | 25    |
| Com_execute_sql | 76    |
| Com_prepare_sql | 50    |
+-----------------+-------+


 SHOW GLOBAL STATUS LIKE 'com_stmt_%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Com_stmt_execute        | 100   |
| Com_stmt_close          | 48    |
| Com_stmt_fetch          | 0     |
| Com_stmt_prepare        | 74    |
| Com_stmt_reset          | 0     |
| Com_stmt_send_long_data | 0     |
| Com_stmt_reprepare      | 24    |
+-------------------------+-------+

RAM is not changing drastically

Explain Plan

+----+-------------+---------------+------------+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------+---------+------------------------------------------------+-------+----------+----------------------------------------------------+
| id | select_type | table         | partitions | type   | possible_keys                                                                                                                                                                                                    | key                                              | key_len | ref                                            | rows  | filtered | Extra                                              |
+----+-------------+---------------+------------+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------+---------+------------------------------------------------+-------+----------+----------------------------------------------------+
|  1 | SIMPLE      | bf_parent     | NULL       | ref    | PRIMARY,base_facili_type_6d98f9_idx                                                                                                                                                                              | base_facili_type_6d98f9_idx                      | 257     | const                                          |     1 |   100.00 | Using index; Using temporary; Using filesort       |
|  1 | SIMPLE      | fcbf_parent   | NULL       | ALL    | NULL                                                                                                                                                                                                             | NULL                                             | NULL    | NULL                                           | 19142 |     3.33 | Using where; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | base_referral | NULL       | ref    | referral_disease_id_ad6e6e71_fk_disease_icd_code,referral_from_facility_id_b9840c9f_fk_facility_id,referral_to_facility_id_81a49e98_fk_facility_id,base_referr_disease_b74959_idx,base_referr_disease_9bbdaf_idx | referral_disease_id_ad6e6e71_fk_disease_icd_code | 257     | capacity_planning.fcbf_parent.disease_id       |     3 |   100.00 | Using where                                        |
|  1 | SIMPLE      | bf_current    | NULL       | eq_ref | PRIMARY,base_facili_type_6d98f9_idx                                                                                                                                                                              | PRIMARY                                          | 4       | capacity_planning.base_referral.to_facility_id |     1 |    57.14 | Using where                                        |
|  1 | SIMPLE      | fcbf          | NULL       | ALL    | NULL                                                                                                                                                                                                             | NULL                                             | NULL    | NULL                                           | 19142 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | ir            | NULL       | ALL    | forecast_intermediat_disease_id_689b0572_fk_disease_i                                                                                                                                                            | NULL                                             | NULL    | NULL                                           |     1 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+---------------+------------+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------------------------------------+---------+------------------------------------------------+-------+----------+----------------------------------------------------+

Answer 1

I was finally able to reduce the time dramatically by doing these 3 steps:

• removed excess of indexes
• remove autocommit, foreign key checks and unique checks at the beginning of each procedure
• Changing storage engine of the tables i'm filling to MyISAM.

Answer 2

Your com_prepare_sql was 50, com_dealloc_sql was 25, meaning you have left RESOURCES in USE because the CLOSE() has not signaled you have completed the process.

Your com_stmt_prepare was 72, com_stmt_close was 48, meaning you have left RESOURCES in USE because the CLOSE() has not signaled you have completed the process.

Consistent query execution speed is more likely to be achieved when the dealloc and close counts are closer to the respective prepared counts.

General Log capture and analysis will confirm the resources were left in use and not deallocated or closed.

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