MDEV-39791: Handle count aggregate optimization for replay purpose

[bsrikanth-mariadb]

Separately dump into the optimizer context, the count aggregation value
of any one non-nullable column of a table if any are used in the query.
This is only suppoerted for myisam tables.

During opt_sum_query() call, record the count aggregation values into
the optimizer context recorder. Once the context is being stored into
the information_schema table, dump all these recorded values into it.

During replay, parse the count aggregate values and store them in memory.
Later when opt_sum_query() requires the count value, hook the recorded
value from the parsed context.

[gemini-code-assist]

Code Review

This pull request introduces support for recording and replaying count aggregate contexts (count_agg) within the optimizer context store/replay framework. While the implementation successfully integrates the new structures and serialization routines, several critical issues must be addressed. Most notably, there is a type confusion vulnerability in opt_sum.cc where a cast to Item_field* is performed without verifying the item type, which could lead to server crashes. Additionally, there are bugs involving the serialization of a pointer instead of the actual count value, incorrect string appending of a ulong counter, potential counter desynchronization during OOM events, and swapped error messages in the JSON parsing routines.

[gemini-code-assist]

If get_table_context or the allocation of count_agg_ctx fails (e.g., due to OOM), record_count_agg returns early without incrementing count_agg_counter. However, during replay, infuse_count_agg increments count_agg_counter unconditionally. This will cause the counters to get out of sync for any subsequent count aggregates. Incrementing count_agg_counter at the very beginning of record_count_agg ensures that the counters remain synchronized even if recording fails for a specific call.

void Optimizer_context_recorder::record_count_agg(TABLE *tbl, longlong count)
{
  count_agg_counter++;
  table_context_for_store *table_ctx=
      get_table_context(tbl->pos_in_table_list);

  if (unlikely(!table_ctx))
    return; // OOM

  auto *count_agg_ctx= new (mem_root) count_agg;
  if (unlikely(!count_agg_ctx))
    return; // OOM

  count_agg_ctx->call_number= count_agg_counter;
  count_agg_ctx->count= count;
  table_ctx->count_agg_list.push_back(count_agg_ctx, mem_root);
}

[bsrikanth-mariadb]

changed the logic. this is not relevant.

[spetrunia]

MariaDB [test]> set optimizer_record_context=1;
Query OK, 0 rows affected (0.001 sec)

MariaDB [test]> create table t10 (a int not null) engine=myisam;
Query OK, 0 rows affected (0.014 sec)

MariaDB [test]> insert into t10 select seq from seq_1_to_3;
Query OK, 3 rows affected (0.004 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> explain select count(*) from t10;
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.001 sec)

^ this is not caught by capture at all.
This EXPLAIN output means that the query did use get_exact_record_count...

To check that I do have your patch, etc:

MariaDB [test]> explain select count(a) from t10;

Now, this does hits a breakpoint in Optimizer_context_recorder::record_count_agg, as expected.

[spetrunia]

count_agg
record_count_agg
infuse_count_agg

This is a bad name. Please change to exact_row_count .

[spetrunia]

So, there is a call above:

 cnt_item->make_const((longlong) count);

which saves the value of count into the Item_sum_count.
And then, now you do infuse_count_agg . Isn't that too late?

[spetrunia]

This should not have been in your patch. Why put it here?
This is an unrelated failure, I've fixed it in 1ae5cfb (now pushed).

[spetrunia]

Typo, dupicate.

[spetrunia]

Garbage, remove?

[spetrunia]

Please fix the above