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Add PPL and SQL section (#1111) 
* Merge pull request #1 from Yury-Fridlyand/dev-update-sql-relevance-docs

Update SQL plugin relevance functions documentation.

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Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>
Co-authored-by: Yury Fridlyand <yuryf@bitquilltech.com>
Co-authored-by: MaxKsyunz <maxk@bitquilltech.com>
Co-authored-by: Fanit Kolchina <kolchfa@amazon.com>
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default Identifiers SQL and PPL 6

Identifiers

An identifier is an ID to name your database objects, such as index names, field names, aliases, and so on. OpenSearch supports two types of identifiers: regular identifiers and delimited identifiers.

Regular identifiers

A regular identifier is a string of characters that starts with an ASCII letter (lower or upper case). The next character can either be a letter, digit, or underscore (_). It can't be a reserved keyword. Whitespace and other special characters are also not allowed.

OpenSearch supports the following regular identifiers:

  1. Identifiers prefixed by a dot . sign. Use to hide an index. For example .opensearch-dashboards.
  2. Identifiers prefixed by an @ sign. Use for meta fields generated by Logstash ingestion.
  3. Identifiers with hyphen - in the middle. Use for index names with date information.
  4. Identifiers with star * present. Use for wildcard match of index patterns.

For regular identifiers, you can use the name without any back tick or escape characters. In this example, source, fields, account_number, firstname, and lastname are all identifiers. Out of these, the source field is a reserved identifier.

SELECT account_number, firstname, lastname FROM accounts;

| account_number | firstname | lastname | :--- | :--- | | 1 | Amber | Duke
| 6 | Hattie | Bond | 13 | Nanette | Bates | 18 | Dale | Adams

Delimited identifiers

A delimited identifier can contain special characters not allowed by a regular identifier. You must enclose delimited identifiers with back ticks (``). Back ticks differentiate the identifier from special characters.

If the index name includes a dot (.), for example, log-2021.01.11, use delimited identifiers with back ticks to escape it `log-2021.01.11`.

Typical examples of using delimited identifiers:

  1. Identifiers with reserved keywords.
  2. Identifiers with a . present. Similarly, - to include date information.
  3. Identifiers with other special characters. For example, Unicode characters.

To quote an index name with back ticks:

source=`accounts` | fields `account_number`;
account_number
1
6
13
18

Case sensitivity

Identifiers are case sensitive. They must be exactly the same as what's stored in OpenSearch.

For example, if you run source=Accounts, you'll get an index not found exception because the actual index name is in lower case.