Docs should describe in prose what an "explainable statement" is

[rmloveland]

Richard Loveland (rmloveland) commented:

We mention "explainable statements" in the following places in our docs (19.2 folder):

ack 'explainable'
admin-ui-statements-page.md
69:The **Logical Plan** section displays CockroachDB's query plan for an [explainable statement](https://www.cockroachlabs.com/docs/stable/sql-grammar.html#preparable_stmt). You can then use this information to optimize the query. For more information about logical plans, see [`EXPLAIN`](https://www.cockroachlabs.com/docs/stable/explain.html).

explain-analyze.md
26:`preparable_stmt` | The [statement](sql-grammar.html#preparable_stmt) you want details about. All preparable statements are explainable.

explain.md
7:The `EXPLAIN` [statement](sql-statements.html) returns CockroachDB's query plan for an [explainable statement](sql-grammar.html#preparable_stmt). You can then use this information to optimize the query.
45: `preparable_stmt` | The [statement](sql-grammar.html#preparable_stmt) you want details about. All preparable statements are explainable.

table-expressions.md
30:`row_source_extension_stmt` | [Result rows](#using-the-output-of-other-statements) from a supported [explainable statement](sql-grammar.html#row_source_extension_stmt].
47:`[... statement ...)` | [Use the result rows](#using-the-output-of-other-statements) of an [explainable statement](sql-grammar.html#preparable_stmt).<br><br>This is a CockroachDB extension. | `[SHOW COLUMNS FROM accounts]`
402:- [Explainable Statements](sql-grammar.html#preparable_stmt)

I can read the syntax diagrams and deduce "Explainable" seems to mean "stuff that is not SET" or similar.

However, if we are going to use this term we should define it in a more meaningful and understandable way, IMO.

Estimated scope of work:

• Determine whether we are happy with "explainable statement" (i.e., whether to keep it or not. We probably should, at least for EXPLAIN docs)
• Figure out what it actually means besides "things in this part of the syntax diagram"
• Write it up and find a good home for that writeup

Jira Issue: DOC-329