DDL tests for simulated DBI

[krlmlr]

Methods like db_compute() and db_copy_to() currently require a real database to be tested against. Suggesting the following approach:

1. Define a db_execute() generic, implement it only for the "TestConnection" class (which would print the SQL) and for "DBIConnection" (which forwards to DBI::dbExecute())
2. Replace all calls to DBI::dbExecute() with calls to db_execute()
3. Run tests with simulate_dbi(...), capture output

Reference: https://github.com/tidyverse/dbplyr/pull/280/files#r275091349

[hadley]

If we going to rearchitect stuff, I think it would be better to extract out a separate method that just generates the SQL.

[hadley]

Looks like these are the main generics that generate then execute SQL:

• db_explain
• db_analyze
• db_save_query
• db_create_index — but only used by MySQL and now I don't see why*
• db_drop_table

I'm going to add matching sql_*() generics, and change the default db_*() to call use sql_* + dbExecute().

[hadley]

db_save_query() is a little trickier because it has to return the name, and MS SQL needs to mangle it. So probably needs another new generic. (Which I think SAP HANA will also use)