infoschema, util: refresh statement summary table periodically (#13680)

[djshow832]

conflicting files:
global_vars_cache.go
set.go
statement_summary_test.go
sysvar.go
tidb_vars.go
varsutil.go
varsutil_test.go

What problem does this PR solve?

Refresh the table events_statements_summary_by_digest periodically.

What is changed and how it works?

If the current summary exists for over tidb_stmt_summary_refresh_interval seconds, then replace it with a new one.

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

mysql> select * from events_statements_summary_by_digest limit 1\G
*************************** 1. row ***************************
       SUMMARY_BEGIN_TIME: 2019-11-22 13:00:00
                STMT_TYPE: select
              SCHEMA_NAME: performance_schema
                   DIGEST: 688edff0859079e96540c6161b08dde9ab5f8654bd2755a0dbca27de5e06f93e
              DIGEST_TEXT: select * from events_statements_summary_by_digest
              TABLE_NAMES: performance_schema.events_statements_summary_by_digest
              INDEX_NAMES: NULL
              SAMPLE_USER: root@127.0.0.1
               EXEC_COUNT: 1
              SUM_LATENCY: 749473
              MAX_LATENCY: 749473
              MIN_LATENCY: 749473
              AVG_LATENCY: 749473
        AVG_PARSE_LATENCY: 56405
        MAX_PARSE_LATENCY: 56405
      AVG_COMPILE_LATENCY: 444081
      MAX_COMPILE_LATENCY: 444081
             COP_TASK_NUM: 0
     AVG_COP_PROCESS_TIME: 0
     MAX_COP_PROCESS_TIME: 0
  MAX_COP_PROCESS_ADDRESS: NULL
        AVG_COP_WAIT_TIME: 0
        MAX_COP_WAIT_TIME: 0
     MAX_COP_WAIT_ADDRESS: NULL
         AVG_PROCESS_TIME: 0
         MAX_PROCESS_TIME: 0
            AVG_WAIT_TIME: 0
            MAX_WAIT_TIME: 0
         AVG_BACKOFF_TIME: 0
         MAX_BACKOFF_TIME: 0
           AVG_TOTAL_KEYS: 0
           MAX_TOTAL_KEYS: 0
       AVG_PROCESSED_KEYS: 0
       MAX_PROCESSED_KEYS: 0
        AVG_PREWRITE_TIME: 0
        MAX_PREWRITE_TIME: 0
          AVG_COMMIT_TIME: 0
          MAX_COMMIT_TIME: 0
   AVG_GET_COMMIT_TS_TIME: 0
   MAX_GET_COMMIT_TS_TIME: 0
  AVG_COMMIT_BACKOFF_TIME: 0
  MAX_COMMIT_BACKOFF_TIME: 0
    AVG_RESOLVE_LOCK_TIME: 0
    MAX_RESOLVE_LOCK_TIME: 0
AVG_LOCAL_LATCH_WAIT_TIME: 0
MAX_LOCAL_LATCH_WAIT_TIME: 0
           AVG_WRITE_KEYS: 0
           MAX_WRITE_KEYS: 0
           AVG_WRITE_SIZE: 0
           MAX_WRITE_SIZE: 0
     AVG_PREWRITE_REGIONS: 0
     MAX_PREWRITE_REGIONS: 0
            AVG_TXN_RETRY: 0
            MAX_TXN_RETRY: 0
            BACKOFF_TYPES: NULL
                  AVG_MEM: 0
                  MAX_MEM: 0
        AVG_AFFECTED_ROWS: 0
               FIRST_SEEN: 2019-11-22 13:22:53
                LAST_SEEN: 2019-11-22 13:22:53
        QUERY_SAMPLE_TEXT: select * from events_statements_summary_by_digest

Code changes

• Has exported function/method change

Side effects

• Possible performance regression
• Increased code complexity

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release note

• Refresh the system table events_statements_summary_by_digest periodically.

[djshow832]

/run-all-tests

[djshow832]

/run-unit-test

[djshow832]

/run-all-tests

[AilinKid]

LGTM

[bb7133]

LGTM