The vacuum_cost_delay setting affects both the VACUUM and ANALYZE commands in PostgreSQL. According to the PostgreSQL documentation, during the execution of both commands, the system maintains an internal counter that tracks the estimated cost of various I/O operations. When this accumulated cost reaches a specified limit (defined by vacuum_cost_limit), the process will sleep for a short period of time, as specified by vacuum_cost_delay, before resetting the counter and continuing execution.

This feature is designed to reduce the I/O impact of these maintenance commands on concurrent database activity. It is particularly important that these commands do not significantly interfere with the ability of the system to perform other database operations.

To summarize:

• vacuum_cost_delay affects both VACUUM and ANALYZE commands.

• It helps manage the I/O impact of these commands on the database system.

For more details, you can refer to the documentation: PostgreSQL: Documentation: 13: Resource Consumption.

Let's check source code

The source code for the vacuum_delay_point function in PostgreSQL shows how the vacuum_cost_delay is implemented and utilized during the execution of the VACUUM and ANALYZE commands. Here are some key excerpts from the code:

1. Function Definition:

   void vacuum_delay_point(void)
   {
       double msec = 0;
   
       /* Always check for interrupts */
       CHECK_FOR_INTERRUPTS();
   
       if (!VacuumCostActive || InterruptPending)
           return;
   
       /* For parallel vacuum, the delay is computed based on the shared cost balance. */
       if (VacuumSharedCostBalance != NULL)
           msec = compute_parallel_delay();
       else if (VacuumCostBalance >= VacuumCostLimit)
           msec = VacuumCostDelay * VacuumCostBalance / VacuumCostLimit;
   
       /* Nap if appropriate */
       if (msec > 0)
       {
           if (msec > VacuumCostDelay * 4)
               msec = VacuumCostDelay * 4;
   
           pgstat_report_wait_start(WAIT_EVENT_VACUUM_DELAY);
           pg_usleep((long) (msec * 1000));
           pgstat_report_wait_end();
   
           VacuumCostBalance = 0;
   
           /* update balance values for workers */
           AutoVacuumUpdateDelay();
   
           /* Might have gotten an interrupt while sleeping */
           CHECK_FOR_INTERRUPTS();
       }
   }

2. Cost-Based Delay Calculation:

   • The function checks if the cost-based vacuuming is active and whether the accumulated cost has reached the limit. If so, it calculates the delay based on the current cost balance and the configured vacuum_cost_delay.

3. Sleep Mechanism:

   • If the calculated delay (msec) is greater than zero, the function will cause the process to sleep for that duration, effectively throttling the I/O operations to reduce their impact on concurrent database activities.

You can view the complete source code for this function in the PostgreSQL repository:

• PostgreSQL 13 - vacuum.c

• PostgreSQL 15 - vacuum.c

• PostgreSQL 16 - vacuum.c

This confirms that vacuum_cost_delay indeed affects both VACUUM and ANALYZE operations, as it is integrated into the delay mechanism for these commands.

How exactly looking at this source code you conclude that this affects ANALYZE too?

The conclusion that vacuum_cost_delay affects both VACUUM and ANALYZE commands is supported by the source code in the vacuum.c file, specifically in the ExecVacuum function. This function serves as the primary entry point for both manual VACUUM and ANALYZE commands.

Key Points from the Source Code

1. Functionality for Both Commands: The comment at the beginning of the vacuum.c file states:

   /* This file includes (a) control and dispatch code for VACUUM and ANALYZE commands... */

2. ExecVacuum Function: The ExecVacuum function handles the execution of both commands. It parses options that are common to both VACUUM and ANALYZE:

   /* Parse options list */
   foreach(lc, vacstmt->options)
   {
       ...
       else if (!vacstmt->is_vacuumcmd)
           report(ERROR, ...);
       ...
       else if (strcmp(opt->defname, "analyze") == 0)
           analyze = defGetBoolean(opt);
       ...
   }

3. Calling the Vacuum Function: The ExecVacuum function eventually calls the vacuum function, which is responsible for the actual processing:

   vacuum(vacstmt->rels, &params, NULL, isTopLevel);

4. Cost-Based Delay in Vacuum Processing: The vacuum_delay_point function is called within the loop of the vacuum processing, which is applicable to both VACUUM and ANALYZE:

   /* This should be called in each major loop of VACUUM processing */
   void vacuum_delay_point(void) { ... }

Conclusion

The source code indicates that the vacuum_delay_point function, which implements the cost-based delay mechanism, is invoked during the processing of both VACUUM and ANALYZE commands. Therefore, the vacuum_cost_delay setting indeed affects both operations.

For further details, you can refer to the source code:

• PostgreSQL 15 - vacuum.c

• PostgreSQL 16 - vacuum.c

Thanks

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