选择 * 与选择 *

发布于 2024-11-28 00:59:33 字数 376 浏览 0 评论 0原文

昨天，一位同事向我展示了以下 postgres 查询。我们都对它的工作感到惊讶：

SELECT* FROM mytable;

由于我最近为另一种语言编写了一个解析器，我试图更深入地理解为什么这个查询“编译”并返回与 SELECT * FROM mytable; 相同的结果。

据推测，这被识别为有效查询，因为在词法分析期间，postgres 从输入中读取 SELECT 作为标记，然后搜索下一个标记，它发现该标记为 *，等等——这或多或少是这里发生的事情吗？

另外，postgres 词法分析器/解析器是否恰好足够强大以理解此查询，或者其他数据库是否会理解类似的 SELECT* 查询？

原文

Yesterday a colleague showed me the following postgres query. We were both surprised that it worked:

SELECT* FROM mytable;

Since I recently coded a parser for another language, I am trying to understand in more depth why this query "compiles" and returns the same results as SELECT * FROM mytable;.

Presumably this is recognized as a valid query because during lexical analysis, postgres reads SELECT from input as a token, and then searches for the next token, which it finds as *, and so on - is that more or less what is going on here?

Also, does the postgres lexer/parser just happen to be robust enough to understand this query, or would other databases understand a similar SELECT* query?

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一梦等七年七年为一梦 2024-12-05 00:59:33

通常词法分析器会向当前标记添加字符，直到找到不属于当前标记的字符，然后退出并从之前无法继续的位置重新开始。

所以这里发生的事情是词法分析器吞噬了 SELECT 并发现下一个字符是 * ，因为它正在收集一个单词，所以不能属于 >选择。因此它会停止，分析结果是关键字的 SELECT，然后从它识别的 * 开始，依此类推。这与其他编程语言中的 2*2 和 2 * 2 得到 4 的原因相同。

至于是否能在其他数据库上工作，这完全取决于词法分析器的细节和语法的规则。

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风轻花落早 2024-12-05 00:59:33

显然，分词器对算术中使用的空格和特殊字符进行分词。

以下是 SELECT 语句的 BNF：h2database.com：

SELECT [ TOP term ] [ DISTINCT | ALL ] selectExpression [,...]
FROM tableExpression [,...] [ WHERE expression ]
[ GROUP BY expression [,...] ] [ HAVING expression ]
[ { UNION [ ALL ] | MINUS | EXCEPT | INTERSECT } select ] [ ORDER BY order [,...] ]
[ LIMIT expression [ OFFSET expression ] [ SAMPLE_SIZE rowCountInt ] ]
[ FOR UPDATE ]

Apparently, the tokenizer tokenizes on white space and special characters used in arithmetic.

Here's the BNF of SELECT statements: h2database.com:

SELECT [ TOP term ] [ DISTINCT | ALL ] selectExpression [,...]
FROM tableExpression [,...] [ WHERE expression ]
[ GROUP BY expression [,...] ] [ HAVING expression ]
[ { UNION [ ALL ] | MINUS | EXCEPT | INTERSECT } select ] [ ORDER BY order [,...] ]
[ LIMIT expression [ OFFSET expression ] [ SAMPLE_SIZE rowCountInt ] ]
[ FOR UPDATE ]

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