On 28.03.2025 15:20, Ilia Evdokimov wrote:
Then we need to decide clearly what exactly to display in EXPLAIN for
the Memoize node: absolute values (estimated distinct keys and
estimated cache capacity) or ratios (hit_ratio and evict_ratio).
Ratios have the advantage of quickly reflecting the overall
effectiveness of Memoize. However, absolute values have a significant
advantage as they explicitly reveal the reason of Memoize's poor
performance, making problem diagnosis simpler.
With absolute values, users can directly understand the underlying
reason for poor performance. For example: insufficient memory
(capacity < distinct keys), inaccurate planner statistics (distinct
keys significantly different from actual values), poorly ordered keys
(capacity ~ distinct keys, but frequent evictions as seen in the
Evictions parameter), or Memoize simply not being beneficial (capacity
~ distinct keys ~ calls). Ratios, by contrast, only reflect the final
outcome without clearly indicating the cause or the specific steps
needed to resolve the issue.
Thus, absolute values do more than just inform users that a problem
exists; they provide actionable details that enable users to directly
address the problem (increase work_mem, refresh statistics, create
extended statistics, or disable Memoize entirely). Additionally, no
other plan nodes in PostgreSQL currently use a similar ratio-based
approach - everywhere else absolute values are consistently shown
(e.g., number of rows, buckets, batches, memory used, etc.). Using
absolute values in Memoize maintains consistency with existing practice.
I've updated the patch to v5, since the new parameter est_unique_keys
in make_memoize() is now placed near est_entries, which is more
logical and readable than putting it at the end.
Any thoughts?
--
Best Regards,
Ilia Evdokimov,
Tantor Labs LLC.
With the feature freeze coming up soon, I’d like to ask: do we plan to
include this patch in v18?
Please let me know if there’s anything I can do to help move it forward.
--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.
