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Document max read and write sizes for large objects

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https://github.com/jackc/pgx/issues/1865
This commit is contained in:
Jack Christensen
2024-01-13 10:43:35 -06:00
parent 52f2151422
commit b99e2bb7e0
1 changed files with 8 additions and 0 deletions
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large_objects.go
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@@ -67,6 +67,10 @@ type LargeObject struct {
}
// Write writes p to the large object and returns the number of bytes written and an error if not all of p was written.
//
// Write is implemented with a single call to lowrite. The PostgreSQL wire protocol has a limit of 1 GB - 1 per message.
// See definition of PQ_LARGE_MESSAGE_LIMIT in the PostgreSQL source code. To allow for the other data in the message,
// len(p) should be no larger than 1 GB - 1 KB.
func (o *LargeObject) Write(p []byte) (int, error) {
var n int
err := o.tx.QueryRow(o.ctx, "select lowrite($1, $2)", o.fd, p).Scan(&n)
@@ -82,6 +86,10 @@ func (o *LargeObject) Write(p []byte) (int, error) {
}
// Read reads up to len(p) bytes into p returning the number of bytes read.
//
// Read is implemented with a single call to loread. PostgreSQL internally allocates a single buffer for the response.
// The largest buffer PostgreSQL will allocate is 1 GB - 1. See definition of MaxAllocSize in the PostgreSQL source
// code. To allow for the other data in the message, len(p) should be no larger than 1 GB - 1 KB.
func (o *LargeObject) Read(p []byte) (int, error) {
var res []byte
err := o.tx.QueryRow(o.ctx, "select loread($1, $2)", o.fd, len(p)).Scan(&res)