Timothy Messier
f00af362dc
When applying the limit and order by, the where clause also needs to be
applied to yield the correct results. This can be seen with an example.
Assuming a data set where there are 100 sessions, with 10 sessions
belonging to a user with id `u_user1` and 90 sessions belonging to
`u_user2`. If the constructed query is like:
select *
from
(select public_id from session limit 10) s,
session_list ss
where
s.public_id = ss.public_id
and s.user_id = 'u_user1'
order by create_time asc;
The initial sub-query with the limit could return sessions only
belonging to `u_user2`, then it would apply the where clause and exclude
all of the rows.
Instead if the query is:
with
session_ids as (
select public_id
from sessions as s
where
s.user_id = 'u_user1'
order by create_time asc
limit 10
)
select *
from session_list
where
session_list.public_id in (select * from session_ids)
order by create_time asc;
The CTE applies the where clause ensuring that only sessions for
`u_user1` are used for the final select from the view.
Note that the CTE is use primarily for readability.
This also adds an index on `session (create_time)` to aide the query
planner in performing the order by.
db package
Usage
Just some high-level usage highlights to get you started. Read the godocs for a complete list of capabilities and their documentation.
dialect = postgres.New(postgres.Config{
DSN: connectionUrl},
)
conn, _ := gorm.Open(dialect, &gorm.Config{})
// Db implements both the Reader and Writer interfaces
rw := Db{Tx: conn}
// There are writer methods like: Create, Update and Delete
// that will write Gorm struct to the db. These writer methods
// all support options for writing Oplog entries for the
// caller: WithOplog(yourWrapper, yourMetadata)
// the caller is responsible for the transaction life cycle of the writer
// and if an error is returned the caller must decide what to do with
// the transaction, which is almost always a rollback for the caller.
err = rw.Create(context.Background(), user)
// There are reader methods like: LookupByPublicId,
// LookupByName, SearchBy, LookupBy, etc
// which will lookup resources for you and scan them into your Gorm struct
err = rw.LookupByPublicId(context.Background(), foundUser)
// There's reader ScanRows that facilitates scanning rows from
// a query into your Gorm struct
where := "select * from test_users where name in ($1, $2)"
rows, err := rw.Query(context.Background(), where, []interface{}{"alice", "bob"})
defer rows.Close()
for rows.Next() {
user := db_test.NewTestUser()
// scan the row into your Gorm struct
if err := rw.ScanRows(rows, &user); err != nil {
return err
}
// Do something with the Gorm user struct
}
// DoTx is a writer function that wraps a TxHandler
// in a retryable transaction. You simply implement a
// TxHandler that does your writes and hand the handler
// to DoTx, which will wrap the writes in a retryable
// transaction with the retry attempts and backoff
// strategy you specify via options.
_, err = w.DoTx(
context.Background(),
10, // ten retries
ExpBackoff{}, // exponential backoff
func(w Writer) error {
// the TxHandler updates the user's friendly name
return w.Update(context.Background(), user, []string{"FriendlyName"},
// write oplogs for this update
WithOplog(
InitTestWrapper(t),
oplog.Metadata{
"key-only": nil,
"deployment": []string{"amex"},
"project": []string{"central-info-systems", "local-info-systems"},
},
),
)
})