I am trying to import data incrementally into TiDB using the Syncer tool. I am wondering if I frequently execute DDL statements, will it affect the data synchronizing speed?
Yes, it will. Syncer will first finish executing all the DML statements ahead, and then execute the DDL statements. And Syncer won’t start executing new DML statements until it finishes executing the DDL statements.
Related
The tkprof utility generates the trace file with three types of information which are Parse, Execute and Fetch. Could you please explain what is the difference between these three? What will be counted as Parse and Execute and Fetch?
Thanks in advance for your help.
When you issue a SQL statement, Oracle:
Parses your SQL statement. That means Oracle analyzes the correctness of the syntax, checks the access rights, and creates the execution plan (or takes it from the cache).
Actually executes your SQL statement.
For SELECT statements, Oracle fetches the rows returned by your query. (For INSERT, DELETE, and UPDATE Oracle fetches nothing).
The numbers of these operations is written in the trace.
If we are talking about the performance tuning, the idea is to parse SQL statements once and then keep them in cache, execute them when you need and do not close cursors if you will need them again to reduce number of fetches.
I'm writing a simple database migration code in clojure using the new seancorfield/next.jdbc library.
How do I execute several SQL statements at once? The usecase is that I have a SQL file containing query code for migrating from one version to the next. next.jdbc/execute! only executes one statement as designed.
Whether you can execute multiple statements in a single JDBC operation is database-dependent. Some databases allow multiple statements in a single operation, separated by a semicolon. If the JDBC driver supports it, next.jdbc will also support it.
If your JDBC driver does not support it, you'll need to make multiple execute! calls. Some databases allow you to wrap multiple DDL operations in a transaction (some of them ignore the transaction and commit each DDL operation separately anyway), some databases explicitly disallow a transaction around DDL operations.
I am using Oracle 12C, Needed help to understand,
1) I am facing an issue of several sessions in locked state (LibraryCache). Showing me as Parallel queries are running, but I have not set any Parallel clause in DDL of table object.
Only for migration Indexes are created with Parallel clause
But that is only for create Not understand why it is taking for DML also.
2) Also if I assume if that is the case then while running DML from sqleditor its ExecutionPlan shows me noting as parallel.
Finally got an answer through,
http://blog.tanelpoder.com/2007/06/23/a-gotcha-with-parallel-index-builds-parallel-degree-and-query-plans/
is it possible to have only DDL statements with a transaction? and if so, is it possible to get it rollback to the original state, if any one of the DDL statement fails?
Thanks & Regards,
priya.R
in Oracle a DDL statement implicitely commits before and after the statement. Each statement is an independant transaction and you won't be able to rollback a successful statement.
A DDL statement however is always atomic: it either succeeds entirely or fails entirely.
What does Oracle mean by "statement level atomicity"?
Let's cite a couple of chapters from the Concepts:
Statement-Level Read Consistency
Oracle always enforces statement-level read consistency. This guarantees that all the data returned by a single query comes from a single point in time--the time that the query began. Therefore, a query never sees dirty data nor any of the changes made by transactions that commit during query execution. As query execution proceeds, only data committed before the query began is visible to the query. The query does not see changes committed after statement execution begins.
Statement-Level Rollback
If at any time during execution a SQL statement causes an error, all effects of the statement are rolled back. The effect of the rollback is as if that statement had never been run. This operation is a statement-level rollback.
A SQL statement that fails causes the loss only of any work it would have performed itself. It does not cause the loss of any work that preceded it in the current transaction.
It means that any single SQL statement you run is atomic in nature - it will either succeed completely or fail completely. If your SQL statement fails, and triggers that would have run as a result of that SQL statement will fail as well.
This is a statement about the nature of transactions: a Unit Of Work either succeeds or completes in its entirety. If your transaction comprises two inserts plus an update, and the update statement fails, the database will rollback all three statements. Any recursive SQL or other SQL such as that included in triggers is included in the atomic scope of the transaction. Find out more.
Atomicity means the A in the ACID principles of transaction management.