sql: PlaceholderInfo should use slices indexed by placeholder name instead of maps

[nvanbenschoten]

CockroachDB and PostgreSQL both mandate that all placeholder names are parsable as uint64 values. CockroachDB enforces that this value fits within an int64. Postgres doesn't allow more than 65535 parameters because it uses 16-bit integers for the parameter count in its wire protocol. Furthermore, Postgres does not allow sparse placeholder names:

nathan=# prepare test1 as select $1;
PREPARE
nathan=# prepare test2 as select $2;
ERROR:  could not determine data type of parameter $1

We should adopt this behavior as well.

This strongly implies that we should be storing placeholder names as int16 values instead of string values. More importantly, it implies that PlaceholderInfo should replace its map[string]T maps with slices that can be indexed directly by placeholder name.

This has real performance implications. While running workload init tpcc --warehouses=1000, which performs a large number of batched inserts, I saw that placeholder manipulation was responsible for about 7% of CPU utilization. Most of this time was spent dealing with PlaceholderInfo maps. We should be able to eliminate this overhead almost entirely with a few small changes to how we store placeholders.

Assigning @jordanlewis for triage.

[nvanbenschoten]

Hah, to add insult to injury, these lines are showing up in heap profiles:

cockroach/pkg/sql/conn_executor_prepare.go

Lines 306 to 307 in 26bbb77

	k := strconv.Itoa(i + 1)
	qargs[k] = datum

and

cockroach/pkg/sql/conn_executor_prepare.go

Line 334 in 26bbb77

k := strconv.Itoa(i + 1)

[jordanlewis]

This actually is also a crashing bug in Cockroach:

root@127.0.0.1:58327/defaultdb> prepare y as (select $4::int);
PREPARE

Time: 2.973ms

root@127.0.0.1:58327/defaultdb> execute y(1);
E181228 21:28:21.112462 595 sql/conn_executor.go:661  [n1,client=127.0.0.1:58329,user=root] a SQL panic has occurred while executing "EXECUTE y (1)": the desired type for tree.TypeCheck cannot be nil, use types.Any instead
E181228 21:28:21.112677 595 util/log/crash_reporting.go:202  [n1,client=127.0.0.1:58329,user=root] a panic has occurred!
E181228 21:28:21.114886 595 util/log/crash_reporting.go:476  [n1,client=127.0.0.1:58329,user=root] Reported as error 4a36eccba7ff49008f847e48f7896374
panic: the desired type for tree.TypeCheck cannot be nil, use types.Any instead [recovered]
        panic: panic while executing 1 statements: EXECUTE _ (_); caused by the desired type for tree.TypeCheck cannot be nil, use types.Any instead

goroutine 595 [running]:
github.com/cockroachdb/cockroach/pkg/sql.(*connExecutor).closeWrapper(0xc001025600, 0x714f380, 0xc001679e80, 0x6539060, 0x710ae00)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:675 +0x36d
github.com/cockroachdb/cockroach/pkg/sql.(*Server).ServeConn.func1(0xc001025600, 0x714f380, 0xc001679e80)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:407 +0x61
panic(0x6539060, 0x710ae00)
        /usr/local/Cellar/go/1.11/libexec/src/runtime/panic.go:513 +0x1b9
github.com/cockroachdb/cockroach/pkg/sql/sem/tree.TypeCheck(0x7153480, 0xc000807340, 0xc0004d7320, 0x0, 0x0, 0x0, 0x0, 0xc0004d7320, 0x90)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/sem/tree/type_check.go:260 +0x101
github.com/cockroachdb/cockroach/pkg/sql/sqlbase.SanitizeVarFreeExpr(0x7153480, 0xc000807340, 0x0, 0x0, 0x69f72e3, 0x11, 0xc0004d7320, 0x0, 0x8954a01, 0x0, ...)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/sqlbase/table.go:57 +0x174
github.com/cockroachdb/cockroach/pkg/sql.fillInPlaceholders(0xc0004d7560, 0x8954a39, 0x1, 0xc0017360c0, 0x1, 0x1, 0xc0002e3a80, 0x1, 0x1, 0x0, ...)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/execute.go:44 +0x323
github.com/cockroachdb/cockroach/pkg/sql.(*connExecutor).execStmtInOpenState(0xc001025600, 0x714f440, 0xc0011289f0, 0xc00175e72b, 0xc, 0x7153080, 0xc0011287e0, 0x0, 0x0, 0x0, ...)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:317 +0x1961
github.com/cockroachdb/cockroach/pkg/sql.(*connExecutor).execStmt(0xc001025600, 0x714f440, 0xc0011289f0, 0xc00175e72b, 0xc, 0x7153080, 0xc0011287e0, 0x0, 0x0, 0x0, ...)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:98 +0x33f
github.com/cockroachdb/cockroach/pkg/sql.(*connExecutor).run(0xc001025600, 0x714f380, 0xc001679e80, 0xc000804c38, 0x5400, 0x15000, 0xc000804cd0, 0xc0002e3a90, 0x0, 0x0)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1141 +0x2154
github.com/cockroachdb/cockroach/pkg/sql.(*Server).ServeConn(0xc0008a7200, 0x714f380, 0xc001679e80, 0xc001025600, 0x5400, 0x15000, 0xc000804cd0, 0xc0002e3a90, 0x0, 0x0)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:409 +0xce
github.com/cockroachdb/cockroach/pkg/sql/pgwire.(*conn).serveImpl.func4(0xc0008a7200, 0x714f380, 0xc001679e80, 0xc001025600, 0x5400, 0x15000, 0xc000804cd0, 0xc0002e3a90, 0xc0002e3aa0, 0xc000412f84)
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/pgwire/conn.go:319 +0x81
created by github.com/cockroachdb/cockroach/pkg/sql/pgwire.(*conn).serveImpl
        /Users/jordan/repo/src/github.com/cockroachdb/cockroach/pkg/sql/pgwire/conn.go:318 +0x1033