Implement dbSendQuery(immediate = TRUE)

[krlmlr]

This allows to:

• connect to servers with prepared statements disabled (Can't connection to the server with disabled prepare statements #185)
• send raw multi-statement SQL to the database (Send raw SQL to database backend #213)
• use specialized appliances (error using Bingo cartridge #249)
• ...

[krlmlr]

Proof of concept in the f-272-immediate branch, with two separate PqResultImpl classes. It feels easier to keep both behaviors in one class. We need to

• Memoize sql argument so that PQsendQuery() can be called in bind()
• Have a non-const cache
• Call prepare() later (it can't return a PGresult*)
  • Call it in the constructor or not? Check what RMariaDB does, RSQLite also calls in the constructor
• Test behavior with multiple queries and statements
• Rethink when to call wait_for_data(), and when to query sockets

Exploding join:

names <- letters[1:21]

sql <-
  paste0(
    "SELECT * FROM ",
    paste0("(SELECT 1 AS ", names, " UNION ALL SELECT 2 AS ", names, ") ", names, collapse = " CROSS JOIN\n")
  )
writeLines(sql)
#> SELECT * FROM (SELECT 1 AS a UNION ALL SELECT 2 AS a) a CROSS JOIN
#> (SELECT 1 AS b UNION ALL SELECT 2 AS b) b CROSS JOIN
#> (SELECT 1 AS c UNION ALL SELECT 2 AS c) c CROSS JOIN
#> (SELECT 1 AS d UNION ALL SELECT 2 AS d) d CROSS JOIN
#> (SELECT 1 AS e UNION ALL SELECT 2 AS e) e CROSS JOIN
#> (SELECT 1 AS f UNION ALL SELECT 2 AS f) f CROSS JOIN
#> (SELECT 1 AS g UNION ALL SELECT 2 AS g) g CROSS JOIN
#> (SELECT 1 AS h UNION ALL SELECT 2 AS h) h CROSS JOIN
#> (SELECT 1 AS i UNION ALL SELECT 2 AS i) i CROSS JOIN
#> (SELECT 1 AS j UNION ALL SELECT 2 AS j) j CROSS JOIN
#> (SELECT 1 AS k UNION ALL SELECT 2 AS k) k CROSS JOIN
#> (SELECT 1 AS l UNION ALL SELECT 2 AS l) l CROSS JOIN
#> (SELECT 1 AS m UNION ALL SELECT 2 AS m) m CROSS JOIN
#> (SELECT 1 AS n UNION ALL SELECT 2 AS n) n CROSS JOIN
#> (SELECT 1 AS o UNION ALL SELECT 2 AS o) o CROSS JOIN
#> (SELECT 1 AS p UNION ALL SELECT 2 AS p) p CROSS JOIN
#> (SELECT 1 AS q UNION ALL SELECT 2 AS q) q CROSS JOIN
#> (SELECT 1 AS r UNION ALL SELECT 2 AS r) r CROSS JOIN
#> (SELECT 1 AS s UNION ALL SELECT 2 AS s) s CROSS JOIN
#> (SELECT 1 AS t UNION ALL SELECT 2 AS t) t CROSS JOIN
#> (SELECT 1 AS u UNION ALL SELECT 2 AS u) u

^{Created on 2021-09-19 by the reprex package (v2.0.1)}

Wait arbitrary time with pg_sleep() .

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