support/db: PoC: Use COPY instead of INSERT in BatchInsertBuilder

[2opremio]

This is proof-of-concept PR which changes BatchInsertBuilder to use COPYinstead of INSERT.

It brings a ~3.5x speedup.

Before:

goos: darwin
goarch: amd64
pkg: github.com/stellar/go/support/db
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
BenchmarkBatchInsertBuilder
BenchmarkBatchInsertBuilder-8   	      76	  14374755 ns/op
PASS

After:

goos: darwin
goarch: amd64
pkg: github.com/stellar/go/support/db
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
BenchmarkBatchInsertBuilder
BenchmarkBatchInsertBuilder-8   	     294	   3939888 ns/op
PASS

TODO:

• Allow ON CONFLICT insertions (this can be done by inserting on a temporary table. This can even speed up things further when indexes are present, since the temporary table doesn't require indexes. Related: services/horizon: Remove by_hash index, replace with index on hash prefix #4087 and Feature Request: Allow Efficient Access to Account "Actions" Where Account is Source #4051).
• Fix tests

If this goes well and we incorporate it we should later evaluate migrating func (q *Q) upsertRows() to use the same approach.

Related #4022

[bartekn]

There are a couple of issues with using COPY especially connected to a DB transaction (linked in a comment). I think we can continue experimenting on this in a separate branch and decide later when we see all the pros and cons.

[bartekn]

Yes, in general BatchInsertBuilder is used inside a DB transaction of a single ledger ingestion, started before the first call to Row(). I explained some problems with COPY in #316 (comment).

[2opremio]

When using a temporary table (so that an insertion suffix can be used) we still get a ~3x speedup:

goos: darwin
goarch: amd64
pkg: github.com/stellar/go/support/db
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
BenchmarkBatchInsertBuilder
BenchmarkBatchInsertBuilder-8   	     216	   5379026 ns/op
PASS

[2opremio]

I've had to move all the COPYing to Exec() so that it happened all at once. Otherwise ingestion would intersperse other sql statements in the middle of the COPY leading to an error (ERROR: unexpected message type 0x50 during COPY from stdin)

This reduces performance, but we are still at a ~2.7x speedup

goos: darwin
goarch: amd64
pkg: github.com/stellar/go/support/db
cpu: Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
BenchmarkBatchInsertBuilder
BenchmarkBatchInsertBuilder-8   	     204	   5994119 ns/op
PASS