最近几年来，地理信息系统无论是在理论上还是应用上都处在一个飞速发展的阶段。 GIS被应用于多个领域的建模和决策支持，如城市管理、区划、环境整治等等，地理信息成为信息时代重要的组成部分之一； “数字地球”概念的提出，更进一步推动了作为其技术支撑的GIS的发展。 与此同时，一些学者致力于相关的理论研究，如空间感知、空间数据误差、空间关系的形式化等等。 这恰好说明了地理信息系统作为应用技术和学科的两个方面，并且这两个方面构成了相互促进的发展过程。
Before the SQLite statement, you can use the “EXPLAIN” keyword or the “EXPLAIN QUERY PLAN” phrase to describe the details of the table.
If omitted EXPLAIN Keyword or phrase, any modification will cause SQLite Statement and returns a query related to the SQLite Information about how the statement operates.
From
EXPLAINAndEXPLAIN QUERY PLANIs used only for interactive analysis and troubleshooting.The details of the output format may vary depending on the version of SQLite.
1.43.1. Grammar ¶
EXPLAIN The syntax is as follows:
EXPLAIN [SQLite Query] EXPLAIN QUERY PLAN The syntax is as follows:
EXPLAIN QUERY PLAN [SQLite Query] 1.43.2. Example ¶
Hypothetical COMPANY The table has the following records:
ID NAME AGE ADDRESS SALARY ---------- ---------- ---------- ---------- ---------- 1 Paul 32 California 20000.0 2 Allen 25 Texas 15000.0 3 Teddy 23 Norway 20000.0 4 Mark 25 Rich-Mond 65000.0 5 David 27 Texas 85000.0 6 Kim 22 South-Hall 45000.0 7 James 24 Houston 10000.0 Now, let’s check SELECT In the statement Explain Use:
sqlite> EXPLAIN SELECT * FROM COMPANY WHERE Salary >= 20000; This will produce the following results:
addr opcode p1 p2 p3 ---------- ---------- ---------- ---------- ---------- 0 Goto 0 19 1 Integer 0 0 2 OpenRead 0 8 3 SetNumColu 0 5 4 Rewind 0 17 5 Column 0 4 6 RealAffini 0 0 7 Integer 20000 0 8 Lt 357 16 collseq(BI 9 Rowid 0 0 10 Column 0 1 11 Column 0 2 12 Column 0 3 13 Column 0 4 14 RealAffini 0 0 15 Callback 5 0 16 Next 0 5 17 Close 0 0 18 Halt 0 0 19 Transactio 0 0 20 VerifyCook 0 38 21 Goto 0 1 22 Noop 0 0 Now, let’s check SELECT The Explain Query Plan in the statement uses:
SQLite> EXPLAIN QUERY PLAN SELECT * FROM COMPANY WHERE Salary >= 20000; order from detail ---------- ---------- ------------- 0 0 TABLE COMPANY