.ram : { *(.ram_code) } > RAM AT > ROM could you explain the grammar in this segment?
时间: 2024-05-26 10:14:00 浏览: 14
This segment is a memory layout specification for the linker in a C or C++ project. It defines a section named `.ram` that contains code from the section named `.ram_code`. The `>` operator specifies where this section should be placed in memory. In this case, it is placed in the RAM memory area, which is typically separate from the read-only memory (ROM) area. The `AT` keyword is used to specify the starting address of the section in memory. In this case, it is specified to be the same as the ROM area. This means that the code in this section will be loaded into RAM at the same address as it is stored in ROM.
相关问题
.ram : { *(.ram_code) } > RAM AT > ROM could you explain the grammar in this segment in detail?
Certainly!
This segment is defining a memory region named "ram" using the following syntax:
```
.ram : {
*(.ram_code)
} > RAM AT > ROM
```
Here's what each part of this syntax means:
`.ram` is the name of the memory region being defined. This name can be anything, but it's common to use names like "ram", "rom", "flash", etc. to match the physical memory type being used.
The colon `:` separates the region name from the memory region's definition.
The curly braces `{}` enclose the list of memory sections that will be placed in this region.
`*(.ram_code)` is a wildcard pattern that matches any section whose name is "ram_code". The `*` means "match any number of characters", and the parentheses `()` group the pattern together.
`>` is a symbol that means "place the following content in the specified memory region".
`RAM` is the memory region where the content will be placed. This is a memory region defined elsewhere in the linker script.
`AT` is a keyword that defines the starting address of the memory region. In this case, the content will start at the beginning of the "RAM" region.
`ROM` is the memory region where the linker will place this entire memory region (in this case, "ram") in the final executable. This is also a memory region defined elsewhere in the linker script.
So, in summary, this segment defines a memory region named "ram" that will contain any sections with the name "ram_code". The content of this region will be placed in the "RAM" memory region starting at the beginning of that region, and the entire "ram" memory region will be placed in the "ROM" memory region in the final executable.
SELECT t.*, c.manage_person_mobile as phone FROM ( SELECT a.biz_date, a.county_name, a.depart_name, a.person_uuid, a.cust_code, a.cust_name, a.terminal_level_name, a.data_gather_type_name, a.sc_type_name, a.sc_score, b.sc_type_name AS sc_type_name_old, b.sc_score AS sc_score_old, a.sc_score - b.sc_score AS up FROM ads_yh_yx_sctwo a LEFT JOIN ads_yh_yx_sctwo b ON a.cust_code = b.cust_code AND b.biz_date = DATE_FORMAT(DATE_SUB(NOW(), INTERVAL 2 DAY), '%Y%m%d') AND b.person_uuid = '00000000001389361632876_5304_371' WHERE a.biz_date = DATE_FORMAT(DATE_SUB(NOW(), INTERVAL 1 DAY), '%Y%m%d') AND a.person_uuid = '00000000001389361632876_5304_371' AND a.sc_type_name <> b.sc_type_name AND (a.sc_score - b.sc_score) <> 0 ) t LEFT JOIN ads_yh_yx_customer_login c ON t.cust_code = c.cust_code; 优化以上代码
优化以上代码可以考虑以下几点:
1. 子查询替换为临时表:将子查询部分的代码提取出来,创建一个临时表,然后在主查询中引用该临时表,这样可以提高查询效率。
```sql
CREATE TEMPORARY TABLE temp_table AS
SELECT
a.biz_date,
a.county_name,
a.depart_name,
a.person_uuid,
a.cust_code,
a.cust_name,
a.terminal_level_name,
a.data_gather_type_name,
a.sc_type_name,
a.sc_score,
b.sc_type_name AS sc_type_name_old,
b.sc_score AS sc_score_old,
a.sc_score - b.sc_score AS up
FROM ads_yh_yx_sctwo a
LEFT JOIN ads_yh_yx_sctwo b ON a.cust_code = b.cust_code
AND b.biz_date = DATE_FORMAT(DATE_SUB(NOW(), INTERVAL 2 DAY), '%Y%m%d')
AND b.person_uuid = '00000000001389361632876_5304_371'
WHERE a.biz_date = DATE_FORMAT(DATE_SUB(NOW(), INTERVAL 1 DAY), '%Y%m%d')
AND a.person_uuid = '00000000001389361632876_5304_371'
AND a.sc_type_name <> b.sc_type_name
AND (a.sc_score - b.sc_score) <> 0;
```
2. 使用索引:对于频繁使用的列,可以考虑创建索引以提高查询效率。根据实际情况,在 `ads_yh_yx_sctwo` 表的 `biz_date`、`person_uuid` 和 `cust_code` 列上创建索引。
```sql
CREATE INDEX idx_biz_date ON ads_yh_yx_sctwo (biz_date);
CREATE INDEX idx_person_uuid ON ads_yh_yx_sctwo (person_uuid);
CREATE INDEX idx_cust_code ON ads_yh_yx_sctwo (cust_code);
```
3. 优化 JOIN 条件:根据实际情况,可以考虑调整 JOIN 的条件,使其更加精确,减少不必要的匹配。
4. 使用 EXPLAIN 分析查询计划:使用 `EXPLAIN` 命令可以帮助分析查询计划,找出潜在的性能问题,并进行相应优化。
以上是一些常见的优化方法,具体的优化策略需要根据实际情况和数据库结构进行调整。
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