Export all collection documents in MongoDB to a JSON file:

mongoexport --db YOUR_DATABASE --collection YOUR_COLLECTION_NAME --out YOUR_OUTPUT_FILE_NAME.json

NOTE: do not use this command for full database backup! Use mongodump instead

#mongodb #export #mongoexport #mongodump #collection #database

In case you want to store special characters inside of MySQL table field, you need to change the table encoding like below:

ALTER TABLE YOUR_TABLE CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci;

#database #mysql #charset #character_set #collate #utf8 #utf8_unicode_ci #alter

To see your current database in MySQL:

mysql> SELECT DATABASE();
 +------------+
 | DATABASE() |
 +------------+
 | my_data    |
 +------------+
 1 row in set (0.00 sec)

As shown above my current database is my_data. I usually forgot what db I'm currently on, when I use tmux and my session has been kept open for weeks. That's why! :)

#mysql #database #current_database #current_db

MongoDB data types

String: You know what it is!

Integer: This type is used to store a numerical value. Integer can be 32 bit or 64 bit depending upon your server.

Boolean: True/False

Double: This type is used to store floating point values.

Arrays: [list, of, elements]

Timestamp: This can be handy for recording when a document has been modified or added.

Object: This datatype is used for embedded documents. Like {"images": {"a": "ali", "b": "reza"}}

Null: This type is used to store a Null value.

Date: This datatype is used to store the current date or time in UNIX time format. You can specify your own date time by creating object of Date and passing day, month, year into it.

Object ID: This datatype is used to store the document’s ID.

There are some more like Code, Regex and so which is used less than other data types.

#mongodb #data_type #mongo #database #collection #object

InnoDB file per table why?
if it is not started and failed what to do?


Today was a big day as a technical point of view in MySQL that saved a lot of storage for me and great deal of maintenance in the future.

To better explain the issue I have to talk a little bit about fundamental behaviour of MySQL InnoDB storage engine!

in past MySQL used MyISAM as its default storage engine. It didn't support transaction. It was not fault tolerant and data was not reliable when power outages occured or server got restarted in the middle of the MySQL actions. By now MySQL uses InnoDB as its default storage engine that is battery packed by transactions, fault tolerant and more.

In InngoDB by default all tables and all databases resides in a single gigantic file called ibdata. When data grows and you alter your tables, the scar gets worse! The size of the ibdata grows very fast. When you alter a table ibdata file would not shrink. For example we had a 120GB single file on server that altering a single table with a huge data would take a long time and would take long
storage, our server went out of free space.

There a is mechanism in MySQL that you configure InnoDB to store each tables data into its own file not inside of ibdata file. This mechnism has great advantages like using OPTIMIZE TABLE to shrink table size.

The OPTIMIZE TABLE whith InnoDB tables, locks the table, copy the data in a new clean table (that's why the result is shrinked), drop the original table and rename the new table with the original name. That why you should care to have the double of the volumetry of your table available in your disk. If you have a 30GB table, optimizing that table needs at least 30GB of free disk space.

Do not use optimize table on a table, when you have not configured innodb file per table. Running OPTIMIZE TABLE against an InnoDB table stored ibdata1 will make things worse because here is what it does:

- Makes the table's data and indexes contiguous inside ibdata1.

- It makes ibdata1 grow because the contiguous data is appended to ibdata1.

You can segregate Table Data and Table Indexes from ibdata1 and manage them independently using innodb_file_per_table. That way, only MVCC and Table MetaData would reside in ibdata1.

In the next post I explain how to do exactly that.

#mysql #innodb #myisam #ibdata1 #database #innodb_file_per_table

How to store Emojis like 🙄 in MySQL database?

https://mathiasbynens.be/notes/mysql-utf8mb4

#mysql #emoji #charset #database

Upgrade mongoDB from 3.4 to 3.6:

Here we persume you are on debian 8 jessie.

1- import public key:

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv                        2930ADAE8CAF5059EE73BB4B58712A2291FA4AD5

2- create apt sources file:

echo "deb http://repo.mongodb.org/apt/debian jessie/mongodb-org/3.6 main" | sudo tee /   etc/apt/sources.list.d/mongodb-org-3.6.list

3- update repo

sudo apt-get update

4- install the MongoDB packages

sudo apt-get install -y mongodb-org=3.6.2 mongodb-org-server=3.6.2 mongodb-org-shell=3.  6.2 mongodb-org-mongos=3.6.2 mongodb-org-tools=3.6.2

* it will ask for config overwrite, if you want to take backup take a backup from config and then overwrite it.


#mongodb #mongo #mongodb36 #database #upgrade #mongodb34

Check grants of a specific user on MySQL:

SELECT sql_grants FROM common_schema.sql_show_grants WHERE user='app';

Please make sure that you have permissions on getting grants list, otherwise permission denied will be returned back.

#mysql #grants #sql_grants #database

OperationalError: (2013, 'Lost connection to MySQL server during query')

Usually it indicates network connectivity trouble and you should check the condition of your network if this error occurs frequently. If the error message includes “during query,” this is probably the case you are experiencing.

Sometimes the “during query” form happens when millions of rows are being sent as part of one or more queries. If you know that this is happening, you should try increasing net_read_timeout from its default of 30 seconds to 60 seconds or longer, sufficient for the data transfer to complete.

More rarely, it can happen when the client is attempting the initial connection to the server. In this case, if your connect_timeout value is set to only a few seconds, you may be able to resolve the problem by increasing it to ten seconds, perhaps more if you have a very long distance or slow connection. You can determine whether you are experiencing this more uncommon cause by using SHOW GLOBAL STATUS LIKE 'Aborted_connects'. It will increase by one for each initial connection attempt that the server aborts. You may see “reading authorization packet” as part of the error message; if so, that also suggests that this is the solution that you need.

If the cause is none of those just described, you may be experiencing a problem with BLOB values that are larger than max_allowed_packet, which can cause this error with some clients. Sometime you may see an ER_NET_PACKET_TOO_LARGE error, and that confirms that you need to increase max_allowed_packet.

#database #mysql #OperationalError #connection

In order to connect directly to MongoDB from your host:

mongo YOUR_REMOTE_MONGO_SERVER:27017

If your MongoDB port is different, use your desired port rather than 27017.


The interesting thing about this command is that you can give database name that you want to work on:

mongo YOUR_REMOTE_MONGO_SERVER:27017/YOUR_DB

Now after connecting if you use db command you should see your current db:

rs0:PRIMARY> db
 YOUR_DB

rs0:PRIMARY will be shown when you use replication. Your case may be different.

#database #mongodb #mongo