Update perf best practices with interleave caveats

[rmloveland]

Fixes #3397.

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/ca689c83dc93319c32d840a9357a52ecee043f94/

[knz]

I would write (e.g., SELECT or INSERT) instead of trying to list all of them. As it is now you're forgetting half a dozen already.

[rmloveland]

Fixing now, thanks.

Follow-up question: Does this information also apply to the following versions?

• 2.0.*
• 2.2

If so I'll add this information there as well.

[rmloveland]

Ok, updated this PR so this info is added to the 2.1 and 2.2 docs. Raphael, if you confirm it applies to 2.0, I'll add it there as well.

[knz]

Yes I like this better.

But what of this phrasing "significantly slower"?
What does the word "significantly" mean? Significant according to which standard? Compared to what?

I think it may be useful to explain a little more: when the data is interleaved, queries that work on the parent table(s) will need to "skip over" the data in interleave children. This increases the read and write latencies to the parent in proportion to the number of interleaved values.

(Conversely: if there are only few interleaved values, the performance will not be "significantly" slower.)

[rmloveland]

What does the word "significantly" mean? Significant according to which standard? Compared to what?

😂

Thanks for this additional feedback, Raphael.

I have attempted another revision that includes this information, rewritten slightly for clarity. Please let me know what you think.

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/3e522c8b0b39c7091bcf8db9fdd82ead96c43180/

[knz]

thank you!

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/20a3bb0d8fbb5bafe03457f7d7d9ec18f4a0a445/

[jseldess]

, with a nit.

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @jseldess, @knz, and @rmloveland)

---

v2.1/performance-best-practices-overview.md, line 51 at r2 (raw file):

[Interleaving tables](interleave-in-parent.html) improves query performance by optimizing the key-value structure of closely related tables, attempting to keep data on the same key-value range if it's likely to be read and written together. This is particularly helpful if the tables are frequently joined on the columns that consist of the interleaving relationship.

However, the above is only true for tables where all operations (e.g., [`SELECT`](selection-queries.html) or [`INSERT`](insert.html)) are performed on a single value shared between both tables.  The following types of operations may actually become slower after interleaving:

nit: Use only one space after periods.

[cockroach-teamcity]

http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/fe267ac1ca503c0c6d903ac2146a605d19caba85/