next-jdbc/doc/tips-and-tricks.md

# Tips & Tricks

This page contains various tips and tricks that make it easier to use `next.jdbc` with a variety of databases. It is mostly organized by database, but there are a few that are cross-database and those are listed first.

## CLOB & BLOB SQL Types

Columns declared with the `CLOB` or `BLOB` SQL types are typically rendered into Clojure result sets as database-specific custom types but they should implement `java.sql.Clob` or `java.sql.Blob` (as appropriate). In general, you can only read the data out of those Java objects during the current transaction, which effectively means that you need to do it either inside the reduction (for `plan`) or inside the result set builder (for `execute!` or `execute-one!`). If you always treat these types the same way for all columns across the whole of your application, you could simply extend `next.jdbc.result-set/ReadableColumn` to `java.sql.Clob` (and/or `java.sql.Blob`). Here's an example for reading `CLOB` into a `String`:

```clojure
(extend-protocol rs/ReadableColumn
  java.sql.Clob
  (read-column-by-label [^java.sql.Clob v _]
    (with-open [rdr (.getCharacterStream v)] (slurp rdr)))
  (read-column-by-index [^java.sql.Clob v _2 _3]
    (with-open [rdr (.getCharacterStream v)] (slurp rdr))))
```

There is a helper in `next.jdbc.result-set` to make this easier -- `clob->string`:

```clojure
(extend-protocol rs/ReadableColumn
  java.sql.Clob
  (read-column-by-label [^java.sql.Clob v _]
    (clob->string v))
  (read-column-by-index [^java.sql.Clob v _2 _3]
    (clob->string v)))
```

As noted in [Result Set Builders](/doc/result-set-builders.md), there is also `clob-column-reader` that can be used with the `as-*-adapter` result set builder functions.

No helper or column reader is provided for `BLOB` data since it is expected that the semantics of any given binary data will be application specific. For a raw `byte[]` you could probably use:

```clojure
    (.getBytes v 1 (.length v)) ; BLOB has 1-based byte index!
```

Consult the [java.sql.Blob documentation](https://docs.oracle.com/javase/8/docs/api/java/sql/Blob.html) for more ways to process it.

> Note: the standard MySQL JDBC driver seems to return `BLOB` data as `byte[]` instead of `java.sql.Blob`.

## Handling Timeouts

JDBC provides a number of ways in which you can decide how long an operation should run before it times out. Some of these timeouts are specified in seconds and some are in milliseconds. Some are handled via connection properties (or JDBC URL parameters), some are handled via methods on various JDBC objects.

Here's how to specify various timeouts using `next.jdbc`:

* `connectTimeout` -- can be specified via the "db-spec" hash map or in a JDBC URL, it is the number of **milliseconds** that JDBC should wait for the initial (socket) connection to complete. Database-specific (may be MySQL only?).
* `loginTimeout` -- can be set via `.setLoginTimeout()` on a `DriverManager` or `DataSource`, it is the number of **seconds** that JDBC should wait for a connection to the database to be made. `next.jdbc` exposes this on the `javax.sql.DataSource` object it reifies from calling `get-datasource` on a "db-spec" hash map or JDBC URL string.
* `queryTimeout` -- can be set via `.setQueryTimeout()` on a `Statement` (or `PreparedStatement`), it is the number of **seconds** that JDBC should wait for a SQL statement to complete. Since this is the most commonly used type of timeout, `next.jdbc` exposes this via the `:timeout` option which can be passed to any function that may construct a `Statement` or `PreparedStatement`.
* `socketTimeout` -- can be specified via the "db-spec" hash map or in a JDBC URL, it is the number of milliseconds that JDBC should wait for socket operations to complete. Database-specific (MS SQL Server and MySQL support this, other databases may too).

Examples:

```clojure
;; connectTimeout / socketTimeout via db-spec:
(def db-spec {:dbtype "mysql" :dbname "example" :user "root" :password "secret"
              ;; milliseconds:
              :connectTimeout 60000 :socketTimeout 30000}))

;; socketTimeout via JDBC URL:
(def db-url (str "jdbc:sqlserver://localhost;user=sa;password=secret"
                 ;; milliseconds:
                 ";database=master;socketTimeout=10000"))

;; loginTimeout via DataSource:
(def ds (jdbc/get-datasource db-spec))
(.setLoginTimeout ds 20) ; seconds

;; queryTimeout via options:
(jdbc/execute! ds ["select * from some_table"] {:timeout 5}) ; seconds

;; queryTimeout via method call:
(let [ps (jdbc/prepare ds ["select * from some_table"])]
  (.setQueryTimeout ps 10) ; seconds
  (jdbc/execute! ps))
```

## MS SQL Server

In MS SQL Server, the generated key from an insert comes back as `:GENERATED_KEYS`.

By default, you won't get table names as qualifiers with Microsoft's JDBC driver (you might with the jTDS drive -- I haven't tried that recently). See this [MSDN forum post about `.getTableName()`](https://social.msdn.microsoft.com/Forums/sqlserver/en-US/55e8cbb2-b11c-446e-93ab-dc30658caf99/resultsetmetadatagettablename-returns-instead-of-table-name) for details. According to one of the answers posted there, if you specify `:result-type` and `:concurrency` in the options for `execute!`, `execute-one!`, `plan`, or `prepare`, that will cause SQL Server to return table names for columns. `:result-type` needs to be `:scoll-sensitive` or `:scroll-insensitive` for this to work. `:concurrency` can be `:read-only` or `:updatable`.

## MySQL & MariaDB

In MySQL, the generated key from an insert comes back as `:GENERATED_KEY`. In MariaDB, the generated key from an insert comes back as `:insert_id`.

MySQL generally stores tables as files so they are case-sensitive if your O/S is (Linux) or case-insensitive if your O/S is not (Mac, Windows) but the column names are generally case-insensitive. This can matter when if you use `next.jdbc.result-set/as-lower-maps` because that will lower-case the table names (as well as the column names) so if you are round-tripping based on the keys you get back, you may produce an incorrect table name in terms of case. You'll also need to be careful about `:table-fn`/`:column-fn` because of this.

It's also worth noting that column comparisons are case-insensitive so `WHERE foo = 'BAR'` will match `"bar"` or `"BAR"` etc.

### Streaming Result Sets

You should be able to get MySQL to stream very large result sets (when you are reducing over `plan`) by setting the following options:

* `:fetch-size Integer/MIN_VALUE` -- when running `plan` (or when creating a `PreparedStatement`).

> Note: it's possible that other options may be required as well -- I have not verified this yet -- see, for example, the additional options PostgreSQL requires, below.

## Oracle

Ah, dear old Oracle! Over the years of maintaining `clojure.java.jdbc` and now `next.jdbc`, I've had all sorts of bizarre and non-standard behavior reported from Oracle users. The main issue I'm aware of with `next.jdbc` is that Oracle's JDBC drivers all return an empty string from `ResultSetMetaData.getTableName()` so you won't get qualified keywords in the result set hash maps. Sorry!

## PostgreSQL

When you use `:return-keys true` with `execute!` or `execute-one!` (or you use `insert!`), PostgreSQL returns the entire inserted row (unlike nearly every other database that just returns any generated keys!).

If you have a query where you want to select where a column is `IN` a sequence of values, you can use `col = ANY(?)` with a native array of the values instead of `IN (?,?,?,,,?)` and a sequence of values.

What does this mean for your use of `next.jdbc`? In `plan`, `execute!`, and `execute-one!`, you can use `col = ANY(?)` in the SQL string and a single primitive array parameter, such as `(int-array [1 2 3 4])`. That means that in `next.jdbc.sql`'s functions that take a where clause (`find-by-keys`, `update!`, and `delete!`) you can specify `["col = ANY(?)" (int-array data)]` for what would be a `col IN (?,?,?,,,?)` where clause for other databases and require multiple values.

### Streaming Result Sets

You can get PostgreSQL to stream very large result sets (when you are reducing over `plan`) by setting the following options:

* `:auto-commit false` -- when opening the connection
* `:fetch-size 4000, :concurrency :read-only, :cursors :close, :result-type :forward-only` -- when running `plan` (or when creating a `PreparedStatement`).

### Working with Date and Time

By default, PostgreSQL's JDBC driver does not always perform conversions from `java.util.Date` to a SQL data type.
You can enable this by extending `SettableParameter` to the appropriate (Java) types, or by simply requiring [`next.jdbc.date-time`](https://cljdoc.org/d/seancorfield/next.jdbc/CURRENT/api/next.jdbc.date-time).

In addition, if you want `java.time.Instant`, `java.time.LocalDate`, and `java.time.LocalDateTime` to be automatically converted to SQL data types, requiring `next.jdbc.date-time` will enable those as well (by extending `SettableParameter` for you).

`next.jdbc.date-time` also includes functions that you can call at application startup to extend `ReadableColumn` to either return `java.time.Instant` or `java.time.LocalDate`/`java.time.LocalDateTime` (as well as a function to restore the default behavior of returning `java.sql.Date` and `java.sql.Timestamp`).

### Working with JSON and JSONB

PostgreSQL has good support for [storing, querying and manipulating JSON data](https://www.postgresql.org/docs/current/datatype-json.html). Basic Clojure data structures (lists, vectors, and maps) transform pretty well to JSON data. With a little help `next.jdbc` can automatically convert Clojure data to JSON and back for us.

First we define functions for JSON encoding and decoding. We're using [metosin/jsonista](https://github.com/metosin/jsonista) in these examples but you could use any JSON library, such as [Cheshire](https://github.com/dakrone/cheshire) or [clojure.data.json](https://github.com/clojure/data.json).

```clojure
(require '[jsonista.core :as json])

;; :decode-key-fn here specifies that JSON-keys will become keywords:
(def mapper (json/object-mapper {:decode-key-fn keyword}))
(def ->json json/write-value-as-string)
(def <-json #(json/read-value % mapper))
```

Next we create helper functions to transform Clojure data to and from PostgreSQL Objects
containing JSON:

```clojure
(import '(org.postgresql.util PGobject))

(defn ->pgobject
  "Transforms Clojure data to a PGobject that contains the data as
  JSON. PGObject type defaults to `jsonb` but can be changed via
  metadata key `:pgtype`"
  [x]
  (let [pgtype (or (:pgtype (meta x)) "jsonb")]
    (doto (PGobject.)
      (.setType pgtype)
      (.setValue (->json x)))))

(defn <-pgobject
  "Transform PGobject containing `json` or `jsonb` value to Clojure
  data."
  [^org.postgresql.util.PGobject v]
  (let [type  (.getType v)
        value (.getValue v)]
    (if (#{"jsonb" "json"} type)
      (with-meta (<-json value) {:pgtype type})
      value)))
```

Finally we extend `next.jdbc.prepare/SettableParameter` and `next.jdbc.result-set/ReadableColumn` protocols to make the conversion between clojure data and PGobject JSON automatic:

```clojure
(require '[next.jdbc.prepare :as prepare])
(require '[next.jdbc.result-set :as rs])

;; if a SQL parameter is a Clojure hash map or vector, it'll be transformed
;; to a PGobject for JSON/JSONB:
(extend-protocol prepare/SettableParameter
  clojure.lang.IPersistentMap
  (set-parameter [m s i]
    (.setObject s i (->pgobject m)))

  clojure.lang.IPersistentVector
  (set-parameter [v s i]
    (.setObject s i (->pgobject v))))

;; if a row contains a PGobject then we'll convert them to Clojure data
;; while reading (if column is either "json" or "jsonb" type):
(extend-protocol rs/ReadableColumn
  org.postgresql.util.PGobject
  (read-column-by-label [^org.postgresql.util.PGobject v _]
    (<-pgobject v))
  (read-column-by-index [^org.postgresql.util.PGobject v _2 _3]
    (<-pgobject v)))
```

#### Inserting and Querying JSON

Let's assume we have following table:

``` sql
create table demo (
  id          serial primary key,
  doc_jsonb   jsonb,
  doc_json    json
)
```

We can now insert Clojure data into json and jsonb fields:

```clojure
(require '[next.jdbc :as jdbc])
(require '[next.jdbc.sql :as sql])

(def db { ...db-spec here... })
(def ds (jdbc/get-datasource db))

(def test-map
  {:some-key "some val" :nested {:a 1} :null-val nil :vector [1 2 3]})

(def data1
  {:doc_jsonb test-map
   :doc_json  (with-meta test-map {:pgtype "json"})})

(sql/insert! ds :demo data1)

(def test-vector
    [{:a 1} nil 2 "lalala" []])

(def data2
    {:doc_jsonb test-vector
     :doc_json  (with-meta test-vector {:pgtype "json"})})

(sql/insert! ds :demo data2)
```

And those columns are nicely transformed into Clojure data when querying:

```clojure
(sql/get-by-id ds :demo 1)
=> #:demo{:id 1,
          :doc_json
          {:some-key "some val",
           :nested {:a 1},
           :vector [1 2 3],
           :null-val nil},
          :doc_jsonb
          {:some-key "some val",
           :nested {:a 1},
           :vector [1 2 3],
           :null-val nil}}

(sql/get-by-id ds :demo 2)
=> #:demo{:id 2,
          :doc_json [{:a 1} nil 2 "lalala" []],
          :doc_jsonb [{:a 1} nil 2 "lalala" []]}

;; Query by value of JSON field 'some-key'
(sql/query ds [(str "select id, doc_jsonb::json->'nested' as foo"
                    "  from demo where doc_jsonb::json->>'some-key' = ?")
               "some val"])
=> [{:demo/id 1, :foo {:a 1}}]
```

#### JSON or JSONB?

* A `json` column stores JSON data as strings (reading and writing is fast but manipulation is slow, field order is preserved)
* A `jsonb` column stores JSON data in binary format (manipulation is significantly faster but reading and writing is a little slower)

If you're unsure whether you want to use json or jsonb, use jsonb.

[<: Friendly SQL Functions](/doc/friendly-sql-functions.md) | [Result Set Builders :>](/doc/result-set-builders.md)