consolidate guidance on avoiding full scans; update session vars #18338

taroface · 2024-02-29T23:15:37Z

DOC-9629

Added a section to SQL Performance Best Practices that summarizes how to prevent the optimizer from planning full scans. Preview
This also consolidates existing guidance across the docs into several includes.
Updated descriptions of the disallow_full_table_scans and large_full_scan_rows settings (for the past 3 docs versions) as it looked like the behavior of these settings was updated in sql, opt: support hint to disallow full scan, update coster to avoid full scans cockroach#71317 but this wasn't documented.

github-actions · 2024-02-29T23:15:57Z

Files changed:

src/current/_includes/v22.2/misc/session-vars.md:

src/current/_includes/v23.1/misc/session-vars.md:

src/current/_includes/v23.2/misc/force-index-selection.md:

src/current/_includes/v23.2/misc/session-vars.md:

src/current/_includes/v23.2/sql/disallow-full-table-scans.md:

src/current/_includes/v23.2/sql/no-full-scan.md:

src/current/v23.2/performance-best-practices-overview.md
src/current/_includes/v23.2/misc/force-index-selection.md:

src/current/_includes/v23.2/sql/transactions-limit-rows.md:

src/current/_includes/v24.1/misc/force-index-selection.md:

src/current/_includes/v24.1/misc/session-vars.md:

src/current/_includes/v24.1/sql/disallow-full-table-scans.md:

src/current/_includes/v24.1/sql/no-full-scan.md:

src/current/v24.1/performance-best-practices-overview.md
src/current/_includes/v24.1/misc/force-index-selection.md:

src/current/_includes/v24.1/sql/transactions-limit-rows.md:

src/current/v23.2/cost-based-optimizer.md
src/current/v23.2/performance-best-practices-overview.md
src/current/v23.2/transactions.md
src/current/v24.1/cost-based-optimizer.md
src/current/v24.1/performance-best-practices-overview.md
src/current/v24.1/transactions.md

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rytaft

This looks great, thanks! Sorry for the delay.

There was another support issue where the customer was tuning their queries and was concerned that the EXPLAIN output showed some full table scans. However, this was expected since the customer was testing with empty tables, and therefore the stats did not represent the production environment.

Do you think it makes sense to update this section of the docs to mention that most of the time the optimizer makes the right decision, and if you see a full scan you should (a) make sure that it's actually causing a performance problem, and (b) confirm that the optimizer would actually choose a full scan in the production environment (with correct stats, cluster topology, etc)? Only then should a customer try to use one of these tools to override the optimizer's decision.

If you think that should be a different PR, I'm happy to file a separate docs issue. Let me know what you think. Thanks!

src/current/_includes/v23.2/sql/disallow-full-table-scans.md

src/current/_includes/v23.2/sql/no-full-scan.md

src/current/v23.2/transactions.md

src/current/_includes/v23.2/sql/transactions-limit-rows.md

taroface · 2024-03-26T19:22:51Z

This looks great, thanks! Sorry for the delay.

There was another support issue where the customer was tuning their queries and was concerned that the EXPLAIN output showed some full table scans. However, this was expected since the customer was testing with empty tables, and therefore the stats did not represent the production environment.

Do you think it makes sense to update this section of the docs to mention that most of the time the optimizer makes the right decision, and if you see a full scan you should (a) make sure that it's actually causing a performance problem, and (b) confirm that the optimizer would actually choose a full scan in the production environment (with correct stats, cluster topology, etc)? Only then should a customer try to use one of these tools to override the optimizer's decision.

If you think that should be a different PR, I'm happy to file a separate docs issue. Let me know what you think. Thanks!

TFTR @rytaft! I had a follow-up question (above).

I do think that a separate docs issue with some more details would make sense for the other recommendations you describe.

rytaft · 2024-03-28T19:25:06Z

This looks great, thanks! Sorry for the delay.
There was another support issue where the customer was tuning their queries and was concerned that the EXPLAIN output showed some full table scans. However, this was expected since the customer was testing with empty tables, and therefore the stats did not represent the production environment.
Do you think it makes sense to update this section of the docs to mention that most of the time the optimizer makes the right decision, and if you see a full scan you should (a) make sure that it's actually causing a performance problem, and (b) confirm that the optimizer would actually choose a full scan in the production environment (with correct stats, cluster topology, etc)? Only then should a customer try to use one of these tools to override the optimizer's decision.
If you think that should be a different PR, I'm happy to file a separate docs issue. Let me know what you think. Thanks!

TFTR @rytaft! I had a follow-up question (above).

I do think that a separate docs issue with some more details would make sense for the other recommendations you describe.

Done: #18428

rytaft

LGTM thank you! And sorry for the delays!

mdlinville

Good refactor. I'll go ahead and approve because I don't feel super strongly about any of my feedback.

src/current/_includes/v22.2/misc/session-vars.md

src/current/_includes/v23.2/misc/force-index-selection.md

src/current/_includes/v23.2/misc/session-vars.md

src/current/_includes/v24.1/misc/session-vars.md

src/current/_includes/v24.1/sql/disallow-full-table-scans.md

src/current/_includes/v24.1/sql/transactions-limit-rows.md

src/current/v23.2/transactions.md

taroface requested a review from rytaft February 29, 2024 23:15

exalate-issue-sync bot mentioned this pull request Mar 4, 2024

Consider adding section on guardrails to avoid full scan #18279

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rytaft approved these changes Mar 26, 2024

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rytaft mentioned this pull request Mar 28, 2024

add guidance on using correct stats when tuning queries with EXPLAIN #18428

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taroface added 2 commits March 28, 2024 16:32

consolidate guidance on avoiding full scans; update session vars

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address rytaft feedback

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rytaft approved these changes Mar 28, 2024

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port changes to 24.1

c4ec112

taroface force-pushed the full-scan-guardrails branch from b7c44db to c4ec112 Compare March 28, 2024 20:40

taroface requested a review from mdlinville March 28, 2024 20:40

mdlinville approved these changes Mar 29, 2024

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taroface force-pushed the full-scan-guardrails branch 2 times, most recently from 0f27f7e to 16e9a05 Compare March 29, 2024 19:37

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b4eaf59

taroface force-pushed the full-scan-guardrails branch from 16e9a05 to b4eaf59 Compare March 29, 2024 19:48

taroface merged commit 6ee9ceb into main Mar 29, 2024
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taroface deleted the full-scan-guardrails branch March 29, 2024 19:54

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consolidate guidance on avoiding full scans; update session vars #18338

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consolidate guidance on avoiding full scans; update session vars #18338

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